Investigators believe the female shooter in California, Tashfeen Malik, posted on Facebook during the deadly attack, pledging allegiance to ISIS group leader Abu Bakr al-Baghdadi, US media reported Friday.
One US official familiar with the investigation said Malik had posted on Facebook under an account with a different name. The officials did not explain how they knew Malik was responsible for the post, CNN reported.
Malik and her American husband, Syed Farook, burst into a year-end party in San Bernardino, California Wednesday and opened fire on a roomful of Farook's co-workers, killing 14 and wounding 21.
"At this point we believe they were more self-radicalized and inspired by the group than actually told to do the shooting," an official told the New York Times, which also reported that Malik had pledged allegiance to ISIS in a Facebook posting but that there was no evidence the group directed the woman.
FBI agents have been combing through cellphones and a computer hard drive left behind by the couple to try to establish a motive for the killings.
CNN, quoting officials, earlier said Farook had been in contact with known terror suspects overseas and had become radicalized after marrying Malik in Saudi Arabia last year, although an imam at a local mosque he attended said Farook showed no signs of that.
The Times reported that the FBI had evidence Farook had communicated with extremists domestically and abroad a few years ago.