Twitter is being sued by the widow of an American killed in Jordan, who accuses the social media company of giving a voice to Islamic State.
Tamara Fields, a Florida woman whose husband, Lloyd, died in the November 9 attack on the police training centre in Amman, said Twitter knowingly let the militant Islamist group use its network to spread propaganda, raise money and attract recruits.
She said the San Francisco-based company had until recently given Islamic State, also known as ISIS, an "unfettered" ability to maintain official Twitter accounts.
"Without Twitter, the explosive growth of ISIS over the last few years into the most-feared terrorist group in the world would not have been possible," according to the complaint filed on Wednesday in the federal court in Oakland, California.
Fields accused Twitter of violating the US Anti-Terrorism Act, which allows triple damages for providing material support to terrorists.
Her lawyer said he believed it was the first case in which a social media company was accused of violating that federal law.
The lawsuit may add to the pressure social media companies face to take down posts associated with terrorist groups.
"While we believe the lawsuit is without merit, we are deeply saddened to hear of this family's terrible loss," Twitter said in a statement about the civil lawsuit.
"Violent threats and the promotion of terrorism deserve no place on Twitter and, like other social networks, our rules make that clear."
Fields may face an uphill battle to prove Twitter knew or should have known its technology was helping terrorists.
"We certainly know social media plays an important role in allowing ISIS to recruit foreign fighters," said Jimmy Gurule, a University of Notre Dame law professor and former US Treasury Department official specialising in terrorist financing.
"But at the end of the day, is there a sufficient nexus between ISIS' use of Twitter and acts of terror?" he said.
"I'm not saying you can't show it but it's a real challenge."