Lawyers representing the family of the two San Bernardino attackers have said relatives are "in complete shock" over the shooting.

They said the family had no idea Syed Rizwan Farook and his wife, Tashfeen Malik, were capable of such an attack.

The lawyers warned against jumping to conclusions after the FBI said earlier the attack was being investigated as an "act of terrorism".

Wednesday's mass shooting left 14 people dead and 21 injured.

Tashfeen Malik, 27, and her husband Syed Rizwan Farook, 28, died in a shootout with police after the killings in the southern Californian city, east of Los Angeles.

Lawyers David Chesley and Mohamed Abuershaid said there was no evidence that the couple had extremist views or were members of a militant group.

Syed Rizwan Farook's sister, Saira Khan, told CBS News: "I can never imagine my brother or my sister-in-law doing something like this, especially because they were happily married, they had a beautiful six-month-old daughter,"

Farook is said to have been an isolated individual with few friends and Malik has been described by family as a "caring, soft-spoken" housewife.

Chesley said Malik was very conservative. She did not drive or interact with male family members and wore a burka, he said.

The family was aware that Farook owned two handguns and said co-workers had recently made fun of his beard, the attorneys said.

FBI Director James Comey said earlier on Friday that there were indications that the couple had been radicalised.

Comey said they were "potentially inspired" by foreign terror groups.

However, he said there was no evidence they were part of a network.

Comey said the investigation was in its early stages and that there was still "a lot of evidence that doesn't make sense".

Earlier, an FBI spokesman said they were also investigating reports that Malik had pledged allegiance to Islamic State (IS).

She is reported to have posted a message on Facebook in support of IS leader Abu Bakr al-Baghdadi under a different name. The post has since been removed.

After Wednesday's attack at the Inland Regional Center social services agency, bomb equipment, weapons and thousands of rounds of ammunition were found in the couple's home.

FBI spokesman David Bowdich told a news conference that authorities were trying to recover data from two mobile phones found crushed in a waste bin near the shooting scene.

He said "telephonic connections" had been established between the couple and other people of interest to the authorities.

"We have uncovered evidence that has led us to learn of extensive planning," he said.

Investigators are also said to be following up a report that Farook had argued with a colleague at work who denounced the "inherent dangers of Islam".

Police said between 75 and 80 people were attending a party at the centre when the shooting began.

The identities of the victims have since been released by San Bernardino's coroner. The youngest was 26 and the oldest was 60.

One of the first officers on the scene spoke of scenes of "unspeakable" carnage in the centre.

San Bernardino is the deadliest mass shooting in the US since 26 people were killed at a school in Newtown, Connecticut in 2012.

A Reuters/Ipsos poll on Friday showed that 51% of Americans view Muslims living in the United States the same as any other community, while only 14.6% were generally fearful of them.