The female suspect in the California gun attack pledged allegiance to the leader of the Islamic State (IS) group on Facebook, US media reported.
Tashfeen Malik made the post under an account with a different name, US officials are quoted as saying.
Fourteen people were killed and 21 wounded in Wednesday's attack.
Malik and her husband Syed Rizwan Farook, 28, died in shootout with police after the killings at San Bernardino.
The New York Times reported that there was no evidence that IS had directed the couple in the attack.
"At this point we believe they were more self-radicalised and inspired by the group than actually told to do the shooting," the newspaper quoted an official as saying.