Tashfeen Malik, one of the attackers in the US social services centre massacre, pledged allegiance to Isis using a Facebook alias, US officials have said.
They confirmed that Malik made her posts under an alias and deleted them before she and her husband, Syed Farook, killed 14 people on Wednesday at a holiday party for his co-workers.
The Muslim couple were killed hours later in a fierce gunbattle with police.
Malik, 27, was a Pakistani who went to the U. in 2014 on a fiancee visa.
Farook, a 28-year-old restaurant health inspector, was born in Chicago to Pakistani parents and raised in Southern California.
A separate US police official said Malik expressed "admiration" for Isis' leader on Facebook under her alias account.
But the official said there was no sign that anyone affiliated with the Islamic State communicated back with her, and there was no evidence of any operational instructions being conveyed to her.
The two officials were not authorized to discuss the case publicly and spoke on condition of anonymity.
The FBI has been investigating the shooting at a social service center as a potential act of terrorism but had reached no firm conclusions as of Thursday, with authorities cautioning repeatedly that the violence could have stemmed from a workplace grudge or a combination of motives.
Separately, a U.S. intelligence official said on Thursday that Farook had been in contact with known Islamic extremists on social media.