Lawsuit says Meta shares blame in the killing of a federal guard.

Facebook’s parent company, Meta Platforms, has been sued over the 2020 killing of a federal security guard, a move that aims to challenge a federal statute that shields websites and social media platforms from liability for what users post.

The lawsuit, filed on Wednesday by Angela Underwood Jacobs, the guard’s sister, argued that Facebook was responsible for connecting individuals who sought to harm law enforcement officers and sow civil discord. Ms. Jacobs’s brother, Dave Patrick Underwood, who served at a federal building and courthouse in Oakland, Calif., was shot and killed in May 2020 by an Air Force sergeant with antigovernment ties, according to the F.B.I.

The shooting “was the culmination of an extremist plot hatched and planned on Facebook by two men who Meta connected through Facebook’s groups infrastructure and its use of algorithms designed and intended to increase user engagement,” said the complaint, which was filed in Alameda County Superior Court in Alameda, Calif.

The suit is the latest challenge to Section 230 of the Communications Decency Act, a 25-year-old law that shields internet companies and websites from liability for what their users post. Unlike publishers, internet companies or website operators are protected by that law.

In her suit, Ms. Jacobs argued that Facebook had become a breeding ground for extremist content and hosted groups that “openly advocated for violence, discussed tactical strategies, combat medicine and the merits of specific weapons, and shared information about building explosive devices.” The lawsuit also said the company’s recommendation algorithms attracted like-minded antigovernment extremists to these groups, including the men involved in the death of her brother.

The sergeant, Steven Carrillo, has been charged with murder and attempted murder, and the man he drove with to Oakland, Robert Justus, has been charged with aiding and abetting murder and attempted murder. Both have pleaded not guilty.

“We’ve banned more than 1,000 militarized social movements from our platform and work closely with experts to address the broader issue of internet radicalization,” Andy Stone, a Meta spokesman, said in a statement. “These claims are without legal basis.”

Militarized social movements continue to have a presence on Meta’s platforms. On Thursday, one such organization ran ads on Instagram, Meta’s popular photo-sharing platform, to recruit members for “a grass-roots movement that pursues readying individual militiamen.” The group’s account was later removed, the company said.