Officials investigating the San Bernardino, California, attack think that as the shooting was on, the female shooter Tashfeen Malik posted a pledge of allegiance to Islamic State leader Abu Bakr al-Baghdadi on Facebook.

CNN quoted a United States official as saying that Malik's post was made on an account with a different name. The officials did not explain how they knew Malik made the post.

A law enforcement official said it appeared that Wednesday's attack -- which left 14 people dead and 21 wounded before the two attackers, Malik and her husband, Syed Rizwan Farook, were killed in a shootout with police -- may have been inspired by the IS.

But none of the officials said that the IS directed or ordered the attack. "This is looking more and more like self-radicalisation," a law enforcement official said.