Tashfeen Malik pleged her loyalty to Islamic State group leader Abu Bakr al-Baghdadi in a Facebook post, according to US officials.
Malik, 27, was shot dead by police along with her husband Syed Rizwan Farook, 28, after a killing spree at an office party in California on Wednesday.
Investigators later said they were investigating a number of possible motives for the killings, including terrorism.
Now various US news sources have quoted officials as saying Malik declared her allegiance to the so-called Islamic State (IS) terror group – also known as Isil, Isis and Daesh – online.
The broadcaster CNN said Pakistan-born Malik, who was living in Saudi Arabia when she married Farook, posted a statement of support for a leader of Islamic State on a Facebook account using a different name.
The New York Times quoted unnamed federal law enforcement officialas as saying there was no evidence IS directed Malik and Farook to launch the attack.
A source told the newspaper: “At this point we believe they were more self-radicalized and inspired by the group than actually told to do the shooting.”
At this point we believe they were more self-radicalized and inspired by the group than actually told to do the shooting. US official
The post has now been removed from Facebook and it is unclear when federal authorities obtained it.
Officials added that the shooters had tried to destroy hard drives and other electronic devices in the days leading up to the attack, suggesting it may have been carefully planned.
But investigators are said to be looking at whether the shooter had been involved in an argument with a co-worker who denounced the “inherent dangers of Islam”.
Mr Farook, 28, was a US citizen of Pakistani heritage who had posted profiles on Muslim dating websites, and apparently the couple met online.
He told co-workers last year that he was traveling to Saudi Arabia to meet his bride, and both American and Saudi officials have confirmed that he spent more than a week in that country in July 2014.
David Bowdich of the FBI said the couple returned to the US together in July 2014. Malik travelled with a K-1 visa, which allows people to come to the country if they plan to marry an American citizen marry within 90 days.
The couple killed 14 people after opening fire at Farook’s workplace, the Inland Regional Center social services agency in San Bernardino, 60 miles east of Los Angeles.
Mourners hold a candlelit vigil for the 14 victims of the San Bernardino massacre (picture: Reuters)
The attack was the deadliest mass shooting in the United States since the Sandy Hook Elementary School shooting in 2012.
Police said the shooters had two assault-style rifles, two semi-automatic handguns and 1,600 rounds of ammunition in their car, with 12 pipe bombs found in their home.
In this life some people cannot have kids. God gave you a gift of a daughter. And you left that kid behind … What did you achieve? Farhan Khan
They left their six-month-old daughter with relatives before going on the killing spree.
Farook’s brother-in-law, Farhan Khan, told NBC News he had begun legal proceedings to adopt the girl, saying: “You left your 6-month-old daughter.
“In this life some people cannot have kids. God gave you a gift of a daughter. And you left that kid behind … What did you achieve?”
Christian Nwadike, who worked with Farook for five years, told CBS that his co-worker had been different since he came back from Saudi Arabia, saying: “I think he married a terrorist.”