Spokeswomen for Twitter and Google declined to comment on the lawsuit. Representatives for Facebook could not immediately be reached by Reuters on Thursday afternoon.
Syed Rizwan Farook and his wife, Tashfeen Malik, opened fire on a holiday gathering of Farook’s co-workers at a government building in San Bernardino on Dec. 2, 2015, killing 14 people and wounding 22 others.
Farook, the 28-year-old, U.S.-born son of Pakistani immigrants, and Malik, 29, a Pakistani native, died in a shootout with police four hours after the massacre.
Authorities have said the couple was inspired by Islamist militants. At the time, the assault ranked as the deadliest attack by Islamist extremists on U.S. soil since the Sept. 11, 2001, attacks. In June 2016, an American-born gunman pledging allegiance to the leader of Islamic State shot 49 people to death at the Pulse nightclub in Orlando, Florida, before he was killed by police.
In December 2016 the families of three men killed at the nightclub sued Twitter, Google and Facebook in federal court on allegations similar to those in the California lawsuit.
Federal law gives internet companies broad immunity from liability for content posted by their users. A number of lawsuits have been filed in recent years seeking to hold social media companies responsible for terror attacks, but none has advanced beyond the preliminary phases.