A 155-millimeter shell fired during a live-fire demonstration for the 250th anniversary of the Marine Corps at Camp Pendleton on Saturday prematurely detonated, dropping fragments of the shell on a California Highway Patrol vehicle and motorcycle that were part of Vice President JD Vance’s protective detail, according to a patrol report.

No officers were hurt in the mishap, which dropped shrapnel onto the vehicles parked on a ramp to a major freeway that had been ordered closed by Gov. Gavin Newsom. The governor had objected to the plan to fire over the freeway, Interstate 5, and ordered a 17-mile stretch closed — against the guidance of military officials, who had said it was safe for it to remain open.

According to the patrol report, one officer described what sounded like pebbles hitting his motorcycle and the area around him, and two others saw a two-inch piece of shrapnel hit the hood of their patrol vehicle, leaving a small dent. The report says shrapnel was also found on the road near the motorcycle.

Mr. Newsom had warned that the Marine Corps’ plans to fire artillery shells over Interstate 5, the West Coast’s main north-south artery, could pose hazards for motorists on the stretch between Los Angeles and San Diego. The closure he ordered on Saturday caused significant backups on the portion of the interstate, which is used by approximately 80,000 people daily.

“We love our Marines and owe a debt of gratitude to Camp Pendleton, but next time, the vice president and the White House shouldn’t be so reckless with people’s lives for their vanity projects,” Mr. Newsom said in a statement to The New York Times.

Lt. Col. Lindsay Pirek, a spokeswoman for the First Marine Expeditionary Force at Camp Pendleton, said the Corps was aware of the report of a possible airborne detonation, and an investigation was underway.

“We are committed to determining the incident’s root cause and applying findings to future missions,” Colonel Pirek said. The statement did not provide additional detail on the munitions used during the exercise.

A spokesman for Mr. Vance declined to comment and referred reporters to the First Marine Expeditionary Force.

According to the report filed by the California Highway Patrol, the artillery round was fired at 1:46 p.m. from White’s Beach, approximately three-quarters of a mile south of Las Pulgas Road, where the highway patrol officers were parked after escorting Mr. Vance to the event.

The exercise — which the report said was expected to include the firing of approximately 60 155-millimeter shells — was terminated after the round prematurely exploded, the patrol said. Patrol officers checked the interstate for shell fragments but found none, and declared it clear before reopening it to vehicular traffic at 2:20 p.m.

The demonstration was part of a larger exercise marking the Corps’ 250th anniversary, attended by Mr. Vance and Defense Secretary Pete Hegseth. A Marine spokesman accompanying reporters witnessing the exercise was pulled aside by another Marine after the initial volley was fired by M777 howitzers, then told members of the news media that the initial plan for multiple volleys had been scaled back. The spokesman, Lt. Col. Brian Coleman, noted that there had been a lengthy back and forth between officials in the days leading up to the event, but gave no further information.

The decision to fire live artillery shells from the oceanfront training area was described as unusual by an active-duty Marine artillery officer and a former Marine artillery noncommissioned officer who spoke on the condition of anonymity because they feared reprisal. Those Marines said the only howitzer training they had previously observed at Camp Pendleton had taken place at approved artillery ranges on the main side of base, east of the interstate, which they said were a much safer option for training.

A highway patrol official based in the area also described it as an “unusual and concerning situation.” Tony Coronado, the highway patrol’s border division chief, said in a statement, that “it

is highly uncommon for any live-fire or explosive training activity to occur near an active freeway.”

Such mishaps are exceedingly rare, according to an active duty Marine who has spent more than 20 years as an artillery officer. The Marine, who was not authorized to speak publicly, said that the most likely explanation for such a mishap was a fault in the projectile’s fuze — a mechanical or electromechanical device fitted to the nose of the shell that causes it to explode.

The Marines fired howitzer shells across Interstate 5 on Friday evening as a rehearsal, evidently without incident.

The Marine Corps said before the event that the exercise would occur on “approved training ranges and comport with established safety protocols” without the need to close public roads.

“All air, surface and ground movements are scripted and rehearsed in accordance with standard operating procedures and established safety checklists,” the Marines said.

If the government shutdown continues into November, about 42 million low-income people could face severe disruptions to their food stamp benefits, the Agriculture Department warned in a letter to state agencies last week, saying that the federal government would have “insufficient funds.”

More than a dozen states have since warned that food stamp recipients may experience significant delays in obtaining benefits next month, see their aid reduced or not receive assistance at all.

The letter, a copy of which was obtained by The New York Times, said that the Agriculture Department’s Food and Nutrition Service, which operates the food stamp program, known as the Supplemental Nutrition Assistance Program, or SNAP, was exploring contingency plans. But it directed state agencies to pause sending vendors the electronic files typically used to load the benefits for November.

“We’re going to run out of money in two weeks,” Brooke L. Rollins, the agriculture secretary, told reporters at the White House on Thursday. “So you’re talking about millions and millions of vulnerable families, of hungry families that are not going to have access to these programs because of this shutdown.”

In a statement, a White House official said that Democrats “chose to shut down the government knowing that programs like SNAP would soon run out of funds.”

Such a disruption would be the first in recent decades. Benefits have remained available through every shutdown in the last 20 years, said Carolyn Vega, the associate director of policy analysis for Share Our Strength, a nonprofit that supports antipoverty programs.

“We are in uncharted territory,” she said.

Nearly 42 million people participated in the food stamp program in May, the month with the latest available data. They received an average of $188, at a total cost of about $8 billion.

Nutrition policy experts pointed to several other funding sources for the food stamp program, and noted that the Agriculture Department could also provide partial benefits for November. The agency could tap into a contingency fund of about $6 billion, but that would still fall short of covering full benefits for the month.

To fund a federal nutrition program for mothers and children known as WIC, the Trump administration used money collected from customs duties. It is possible that the Agriculture Department could use that same source, known as Section 32, but the account is largely used for school lunch and other child nutrition programs, and is unlikely to be sufficient to fund both food stamps and WIC.

The agency also has regulations on how to reduce benefits based on need and household size. For example, higher-income families could see their November benefits reduced by a larger percentage than lower-income ones.

The Agriculture Department could also turn to a specific interpretation of existing law to justify continuing to fund food stamps, said David A. Super, a law professor at Georgetown University. Under that theory, food stamps are an entitlement program, like Medicare, that is not subject to the annual appropriations process.

“The simplest approach for the U.S.D.A. would be to recognize that language in the Food and Nutrition Act makes SNAP an entitlement independent of appropriations, and continue paying benefits on the strength of that language,” Mr. Super said.

Several states, such as Illinois and New York, have already stated that they cannot provide funding from their own coffers. And at least one state is already warning that October benefits, too, may be affected. Minnesota’s Department of Children, Youth and Families said on Friday that it could not guarantee the availability of benefits before Nov. 1 for new enrollees.

Still, Ms. Vega cautioned against panicking, emphasizing that eligible people should still apply. She called for more clarity and a quick resolution from officials in Washington.

“While a delay is certainly better than not issuing November benefits at all, even that can be really significant to a family that is counting on that money and already has a tight budget,” she said.

If the government shutdown continues into November, about 42 million low-income people could face severe disruptions to their food stamp benefits, the Agriculture Department warned in a letter to state agencies last week, saying that the federal government would have “insufficient funds.”

More than a dozen states have since warned that food stamp recipients may experience significant delays in obtaining benefits next month, see their aid reduced or not receive assistance at all.

The letter, a copy of which was obtained by The New York Times, said that the Agriculture Department’s Food and Nutrition Service, which operates the food stamp program, known as the Supplemental Nutrition Assistance Program, or SNAP, was exploring contingency plans. But it directed state agencies to pause sending vendors the electronic files typically used to load the benefits for November.

“We’re going to run out of money in two weeks,” Brooke L. Rollins, the agriculture secretary, told reporters at the White House on Thursday. “So you’re talking about millions and millions of vulnerable families, of hungry families that are not going to have access to these programs because of this shutdown.”

In a statement, a White House official said that Democrats “chose to shut down the government knowing that programs like SNAP would soon run out of funds.”

Such a disruption would be the first in recent decades. Benefits have remained available through every shutdown in the last 20 years, said Carolyn Vega, the associate director of policy analysis for Share Our Strength, a nonprofit that supports antipoverty programs.

“We are in uncharted territory,” she said.

Nearly 42 million people participated in the food stamp program in May, the month with the latest available data. They received an average of $188, at a total cost of about $8 billion.

Nutrition policy experts pointed to several other funding sources for the food stamp program, and noted that the Agriculture Department could also provide partial benefits for November. The agency could tap into a contingency fund of about $6 billion, but that would still fall short of covering full benefits for the month.

To fund a federal nutrition program for mothers and children known as WIC, the Trump administration used money collected from customs duties. It is possible that the Agriculture Department could use that same source, known as Section 32, but the account is largely used for school lunch and other child nutrition programs, and is unlikely to be sufficient to fund both food stamps and WIC.

The agency also has regulations on how to reduce benefits based on need and household size. For example, higher-income families could see their November benefits reduced by a larger percentage than lower-income ones.

The Agriculture Department could also turn to a specific interpretation of existing law to justify continuing to fund food stamps, said David A. Super, a law professor at Georgetown University. Under that theory, food stamps are an entitlement program, like Medicare, that is not subject to the annual appropriations process.

“The simplest approach for the U.S.D.A. would be to recognize that language in the Food and Nutrition Act makes SNAP an entitlement independent of appropriations, and continue paying benefits on the strength of that language,” Mr. Super said.

Several states, such as Illinois and New York, have already stated that they cannot provide funding from their own coffers. And at least one state is already warning that October benefits, too, may be affected. Minnesota’s Department of Children, Youth and Families said on Friday that it could not guarantee the availability of benefits before Nov. 1 for new enrollees.

Still, Ms. Vega cautioned against panicking, emphasizing that eligible people should still apply. She called for more clarity and a quick resolution from officials in Washington.

“While a delay is certainly better than not issuing November benefits at all, even that can be really significant to a family that is counting on that money and already has a tight budget,” she said.

If the government shutdown continues into November, about 42 million low-income people could face severe disruptions to their food stamp benefits, the Agriculture Department warned in a letter to state agencies last week, saying that the federal government would have “insufficient funds.”

More than a dozen states have since warned that food stamp recipients may experience significant delays in obtaining benefits next month, see their aid reduced or not receive assistance at all.

The letter, a copy of which was obtained by The New York Times, said that the Agriculture Department’s Food and Nutrition Service, which operates the food stamp program, known as the Supplemental Nutrition Assistance Program, or SNAP, was exploring contingency plans. But it directed state agencies to pause sending vendors the electronic files typically used to load the benefits for November.

“We’re going to run out of money in two weeks,” Brooke L. Rollins, the agriculture secretary, told reporters at the White House on Thursday. “So you’re talking about millions and millions of vulnerable families, of hungry families that are not going to have access to these programs because of this shutdown.”

In a statement, a White House official said that Democrats “chose to shut down the government knowing that programs like SNAP would soon run out of funds.”

Such a disruption would be the first in recent decades. Benefits have remained available through every shutdown in the last 20 years, said Carolyn Vega, the associate director of policy analysis for Share Our Strength, a nonprofit that supports antipoverty programs.

“We are in uncharted territory,” she said.

Nearly 42 million people participated in the food stamp program in May, the month with the latest available data. They received an average of $188, at a total cost of about $8 billion.

Nutrition policy experts pointed to several other funding sources for the food stamp program, and noted that the Agriculture Department could also provide partial benefits for November. The agency could tap into a contingency fund of about $6 billion, but that would still fall short of covering full benefits for the month.

To fund a federal nutrition program for mothers and children known as WIC, the Trump administration used money collected from customs duties. It is possible that the Agriculture Department could use that same source, known as Section 32, but the account is largely used for school lunch and other child nutrition programs, and is unlikely to be sufficient to fund both food stamps and WIC.

The agency also has regulations on how to reduce benefits based on need and household size. For example, higher-income families could see their November benefits reduced by a larger percentage than lower-income ones.

The Agriculture Department could also turn to a specific interpretation of existing law to justify continuing to fund food stamps, said David A. Super, a law professor at Georgetown University. Under that theory, food stamps are an entitlement program, like Medicare, that is not subject to the annual appropriations process.

“The simplest approach for the U.S.D.A. would be to recognize that language in the Food and Nutrition Act makes SNAP an entitlement independent of appropriations, and continue paying benefits on the strength of that language,” Mr. Super said.

Several states, such as Illinois and New York, have already stated that they cannot provide funding from their own coffers. And at least one state is already warning that October benefits, too, may be affected. Minnesota’s Department of Children, Youth and Families said on Friday that it could not guarantee the availability of benefits before Nov. 1 for new enrollees.

Still, Ms. Vega cautioned against panicking, emphasizing that eligible people should still apply. She called for more clarity and a quick resolution from officials in Washington.

“While a delay is certainly better than not issuing November benefits at all, even that can be really significant to a family that is counting on that money and already has a tight budget,” she said.

If the government shutdown continues into November, about 42 million low-income people could face severe disruptions to their food stamp benefits, the Agriculture Department warned in a letter to state agencies last week, saying that the federal government would have “insufficient funds.”

More than a dozen states have since warned that food stamp recipients may experience significant delays in obtaining benefits next month, see their aid reduced or not receive assistance at all.

The letter, a copy of which was obtained by The New York Times, said that the Agriculture Department’s Food and Nutrition Service, which operates the food stamp program, known as the Supplemental Nutrition Assistance Program, or SNAP, was exploring contingency plans. But it directed state agencies to pause sending vendors the electronic files typically used to load the benefits for November.

“We’re going to run out of money in two weeks,” Brooke L. Rollins, the agriculture secretary, told reporters at the White House on Thursday. “So you’re talking about millions and millions of vulnerable families, of hungry families that are not going to have access to these programs because of this shutdown.”

In a statement, a White House official said that Democrats “chose to shut down the government knowing that programs like SNAP would soon run out of funds.”

Such a disruption would be the first in recent decades. Benefits have remained available through every shutdown in the last 20 years, said Carolyn Vega, the associate director of policy analysis for Share Our Strength, a nonprofit that supports antipoverty programs.

“We are in uncharted territory,” she said.

Nearly 42 million people participated in the food stamp program in May, the month with the latest available data. They received an average of $188, at a total cost of about $8 billion.

Nutrition policy experts pointed to several other funding sources for the food stamp program, and noted that the Agriculture Department could also provide partial benefits for November. The agency could tap into a contingency fund of about $6 billion, but that would still fall short of covering full benefits for the month.

To fund a federal nutrition program for mothers and children known as WIC, the Trump administration used money collected from customs duties. It is possible that the Agriculture Department could use that same source, known as Section 32, but the account is largely used for school lunch and other child nutrition programs, and is unlikely to be sufficient to fund both food stamps and WIC.

The agency also has regulations on how to reduce benefits based on need and household size. For example, higher-income families could see their November benefits reduced by a larger percentage than lower-income ones.

The Agriculture Department could also turn to a specific interpretation of existing law to justify continuing to fund food stamps, said David A. Super, a law professor at Georgetown University. Under that theory, food stamps are an entitlement program, like Medicare, that is not subject to the annual appropriations process.

“The simplest approach for the U.S.D.A. would be to recognize that language in the Food and Nutrition Act makes SNAP an entitlement independent of appropriations, and continue paying benefits on the strength of that language,” Mr. Super said.

Several states, such as Illinois and New York, have already stated that they cannot provide funding from their own coffers. And at least one state is already warning that October benefits, too, may be affected. Minnesota’s Department of Children, Youth and Families said on Friday that it could not guarantee the availability of benefits before Nov. 1 for new enrollees.

Still, Ms. Vega cautioned against panicking, emphasizing that eligible people should still apply. She called for more clarity and a quick resolution from officials in Washington.

“While a delay is certainly better than not issuing November benefits at all, even that can be really significant to a family that is counting on that money and already has a tight budget,” she said.