Police also say Farook and his wife had enough bullets and bombs to slaughter hundreds when they launched their attack on a Christmas party.

The details came to light as investigators tried to determine whether the rampage that left 14 people dead was terrorism, a workplace grudge or some combination.

Wearing black tactical gear and wielding assault rifles, Farook, a 28-year-old county restaurant inspector, and his wife, Tashfeen Malik, 27, sprayed as many as 75 rounds into a room at a social service centre for the disabled, where Farook’s co-workers had gathered for a holiday banquet Wednesday.

Farook had attended the event but slipped out at some point, then returned in battle dress.

Four hours later and a few kilometres away, the couple died in a furious gunbattle, in which they fired 76 rounds, while 23 law officers unleashed about 380, police say.

On Thursday, Police Chief Jarrod Burguan offered a grim morning-after inventory that suggested Wednesday’s bloodbath could have been far worse.

The couple left behind three rigged-together pipe bombs with a remote-control detonating device that apparently malfunctioned, and they had more than 1600 rounds of ammunition left when police shot and killed them in their rented SUV after an hours-long manhunt, Burguan said.

Also, at a family home in the nearby town of Redlands, authorities found 12 pipe bombs, tools for making more, and over 3000 additional rounds of ammunition, the chief said.

“We don’t know if this was workplace rage or something larger or both,” Attorney-General Loretta Lynch said in Washington, echoing President Barack Obama.

“At this point in time we don’t know the motivation.”

Investigators are trying to determine whether became radicalised and if so, how, and whether he was in contact with any foreign terrorist organisation, said the US intelligence official, who spoke on condition of anonymity because he was not authorised to discuss the matter publicly.

But the official said Farook had been in touch on social media with extremists who were on the FBI’s radar screen.

Wednesday’s attack was the nation’s deadliest mass shooting since the Newtown, Connecticut, school tragedy three years ago that left 26 children and adults dead.

Twenty-one people were wounded before the day was out.

Authorities said the attack was carefully planned.

“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,” David Bowdich, assistant director of the FBI’s Los Angeles office, said as the bureau took over the investigation.

Farook has no known criminal record, Burguan said.

He was born in Chicago to a Pakistani family, was raised in Southern California and worked at San Bernardino County’s Department of Public Health for five years, according to authorities and acquaintances.

The Saudi Embassy said he travelled to Saudi Arabia in the summer of 2014 for nine days.

As for Malik, she came to the US in July 2014 on a Pakistani passport and a fiance visa.

To get the visa, immigrants submit to an interview and biometric and background checks – screening intended to identify anyone who might pose a threat.

Before they went on the rampage, couple dropped off their six-month-old daughter with relatives on Wednesday morning, saying they had a doctor’s appointment, according to Hussam Ayloush, executive director of the Council on American-Islamic Relations.

Co-worker Patrick Baccari said he was sitting at the same banquet table as Farook before Farook suddenly disappeared, leaving his coat on his chair.

Baccari said that when the shooting started, he took refuge in a bathroom and suffered minor wounds from shrapnel slicing through the wall.

“If I hadn’t been in the bathroom, I’d probably be laying dead on the floor,” he said.

AP