President Barack Obama has said it is possible the mass shooting in California was related to terrorism, but that authorities still do not know.

He added that it is possible it was workplace-related or that there were mixed motives.

Mr Obama was speaking in the Oval Office to assure Americans that authorities will get to the bottom of what happened.

He also called for people to wait for facts before making judgments.

Mr Obama said many Americans feel there is nothing they can do about mass violence. But he added: "We all have a part to play."

Mr Obama said the nation must make it harder to carry out violence. But he acknowledged the threat cannot be eliminated completely.

He said it will be important for all Americans, including state legislators, to see what they can do.

Investigators on Thursday were trying to learn why Syed Rizwan Farook and his wife or fiancee, Tashfeen Malik, 27, went on the rampage at the Inland Regional Centre.

The 28-year-old county restaurant inspector and Malik killed 14 people and seriously wounded more than a dozen others in an attack on Wednesday at the social service centre for the disabled before they were gunned down in an SUV a few miles away in a shoot-out with police.

Farook was born in the US to a Pakistani family, was raised in Southern California and had been a San Bernardino County employee for five years, according to authorities and acquaintances.

San Bernardino police chief Jarrod Burguan said he had no information on Malik's background. Relatives said Farook had traveled to Saudi Arabia to meet his wife.

Police and federal agents for a second day searched a home in neighbouring Redlands, about seven miles from the centre.

Investigators did not immediately say if the couple had lived there. Public records show it may be the home of a Farook family member.

Residents told KABC-TV Redlands is a sleepy town and expressed shock that the killers might be their neighbours.

The attackers invaded the centre about 60 miles east of Los Angeles, opening fire in a conference area where county health officials were having an employee banquet. Farook attended the banquet, then left, then returned with murderous intent.

"They came prepared to do what they did, as if they were on a mission," Mr Burguan said.

Co-worker Patrick Baccari said he was sitting at the same table as Farook, who suddenly disappeared. Baccari said that when the shooting started, he took refuge in a bathroom and suffered minor wounds from shrapnel slicing through the wall.

The shooting lasted about five minutes, he said, and when he looked in the mirror he realised he was bleeding.

"If I hadn't been in the bathroom, I'd probably be laying dead on the floor," he said.

Baccari described Farook as reserved and said he showed no signs of unusual behavior. Earlier this year, he travelled to Saudi Arabia, was gone for about a month and returned with a wife, Mr Baccari said.

The couple dropped off their six-month-old daughter with relatives on Wednesday morning, saying they had a doctor's appointment, Hussam Ayloush, executive director of the Council on American-Islamic Relations, said after talking with family.

"We don't know the motives. Is it work, race-related? Is it mental illness? Is it extreme ideology? At this point, it's really unknown to us, and at this point it's too soon to speculate," Mr Ayloush said.

Co-workers told the Los Angeles Times that Farook was a devout Muslim but did not talk about religion at work.

Farhan Khan, who is married to Farook's sister, told reporters that he last spoke to his brother-in-law about a week ago. Mr Khan condemned the violence and said he had "absolutely no idea" why Farook would do such a thing.

Seventeen people were wounded, according to authorities. Ten were sent to hospital in critical condition, and three in serious condition, fire chief Tom Hannemann said.

About four hours after the morning carnage, police hunting for the killers riddled a black SUV with gunfire in a shoot-out two miles from the social services centre.

Farook and Malik were found with assault rifles and semi-automatic handguns, and were wearing "assault-style clothing" with ammunition attached, authorities said.

Three explosive devices - all connected to one another - were found at the social services centre, police said.

Federal authorities said the two assault rifles and two handguns used in the violence had been bought legally, but they did not say how and when they got into the attackers' hands.

Police said they found 12 pipe bomb devices at the California home being searched in connection with the mass shooting as well as hundreds of tools that could be used to make improvised explosive devices.

Mr Burguan said at a news conference that the attackers also left a device at the social service centre where they opened fire. The device consisted of three connected pipe bombs with a remote control that apparently did not work.