The two attackers who killed 14 people in a gun rampage in California fired up to 75 rifle rounds, left behind three pipe bombs and had over 1,600 more bullets with them when they were gunned down.

At their home, they had 12 pipe bombs, tools for making more explosives, and more than 3,000 rounds of ammunition, Police Chief Jarrod Burguan said.

Wearing military-style gear and wielding assault rifles, Syed Rizwan Farook, a 28-year-old county restaurant inspector, and his wife, Tashfeen Malik, 27, killed 14 and wounded 17 in the attack at a social service centre shortly after he slipped away from an employee banquet he was attending there.

The couple were shot to death about four hours later and a few miles away in a furious gun battle with police.

As the FBI took over the investigation, authorities were trying to learn why the couple left behind their six-month-old daughter and went on the rampage - the nation's deadliest mass shooting since the Newtown, Connecticut, school tragedy three years ago that left 26 children and adults dead.

"There was obviously a mission here. We know that. We do not know why. We don't know if this was the intended target or if there was something that triggered him to do this immediately," said David Bowdich, assistant director of the FBI's Los Angeles office.

At the White House, President Barack Obama said after meeting with his national security team that it was "possible this was terrorist-related" but that authorities were unsure. He raised the possibility that it was a workplace dispute or that mixed motives were at play.

Law enforcement experts said investigators may well conclude the killers had more than one motivation.

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. Authorities said Malik came to the US on a Pakistani passport in July 2014.

Police and federal agents for a second day searched a home in neighbouring Redlands, about seven miles from the massacre at the Inland Regional 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 at around 11am, opening fire in a conference area county health officials had rented for an employee banquet.

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.

Co-worker Patrick Baccari said he was sitting at the same table as Farook, who suddenly disappeared. Mr 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.

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

The couple dropped off their 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, rage-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 kill.

Seventeen people were wounded, according to authorities. As of Thursday, two patients were listed in critical condition.

About four hours after the incident, police hunting for the killers riddled a black SUV with gunfire in a shoot-out 2 miles from the social services centre in the Southern California city of 214,000 people.

During the shoot-out, the couple fired 76 rounds, while law enforcement officers unleashed about 380, the police chief said.