The article:

Court and police records show that police began investigating 17-year-old Celeste Burgess and her mother Jessica Burgess after receiving a tip-off that the pair had illegally buried a stillborn child given birth to prematurely by Celeste. The two women told detective Ben McBride of the Norfolk, Nebraska Police Division that they’d discussed the matter on Facebook Messenger, which prompted the state to issue Meta with a search warrant for their chat history and data including log-in timestamps and photos.

From Motherboard (where you also can read court documents):

The state’s case relies on evidence from the teenager’s private Facebook messages, obtained directly from Facebook by court order, which show the mother and daughter allegedly bought medication to induce abortion online, and then disposed of the body of the fetus.

According to court records, Celeste Burgess, 17, and her mother, Jessica Burgess, bought medication called Pregnot designed to end pregnancy. Pregnot is a kit of mifepristone and misoprostol, which is often used to safely end pregnancy in the first trimester. In this case, Burgess was 28-weeks pregnant, which is later in pregnancy than mifepristone and misoprostol are recommended for use. It’s also later than Nebraska’s 20-week post-fertilization abortion ban, which makes allowances only if the pregnant person is at risk of death or "serious risk of substantial and irreversible physical impairment of a major bodily function." (Nebraska’s abortion laws have not changed since Roe v Wade was overturned).