Gunwoman 'Praised IS' During California Attack

According to law enforcement officials, the Pakistan-born 27-year-old pledged loyalty to the extremist organisation's leader, Abu Bakr al-Baghdadi.

She made the declaration on Facebook using an alias, then deleted the message later.

With her spouse, 28-year-old Syed Farook, she massacred 14 of his colleagues and injured another 21 during a holiday party at a social services centre.

Malik, who grew up in Saudi Arabia, came to the US in 2014 under a K1 fiancee visa, sponsored by her American citizen husband.

"At this point we believe they were more self-radicalised and inspired by the group than actually told to do the shooting," a federal law enforcement official told the New York Times.

A family member tells Reuters news agency they have been contacted by Pakistani intelligence as part of the San Bernardino investigation.

US intelligence officials also say that Farook, an Illinois-born restaurant inspector, had been in touch with Islamic extremists on social media.

His brother-in-law, Farhan Khan, has told NBC News he was a "bad person", but he wasn't radical.

The killers - who were killed by police in a car chase - dropped off their six-month-old daughter with relatives on Wednesday morning before the massacre.

Hours later Farook, an employee of the county public health department, stormed out of his work Christmas party after an apparent dispute.

He returned a short time later with Malik - both of them dressed in black tactical gear and wielding weapons - before they sprayed up to 75 rounds at colleagues.

The first police officer to arrive at the scene of the shootings has said there was "unspeakable carnage".

At the killers' home in Redlands, California, investigators found a dozen pipe bombs in a bag and nearly 5,000 rounds of ammunition.

The FBI have been scouring the couple's mobile phones and a computer hard drive.

Farook reportedly had an argument over religion two weeks ago with a Jewish co-worker who died in the attack, but it isn't clear if that was connected to the massacre.

Nicholas Thalasinos' friend, Kuuleme Stephens, told the Associated Press news agency she had overheard the argument during a phone call.

A candlelight prayer vigil for the victims, who ranged in age from 26 to 60, was held on Thursday night at San Bernardino's minor-league baseball park.