California gunman Syed Rizwan Farook had been in contact with known Islamic extremists on social media, a US intelligence official said on Thursday, and police said he and his wife had enough bullets and bombs to slaughter hundreds when they launched their deadly attack on a holiday party.

The details emerged as investigators tried to determine whether the rampage that killed 14 people in San Bernardino was terrorism, a workplace grudge or me combination.

The husband-and-wife killers were not under FBI scrutiny before the massacre, said a second US official, who likewise was not authorized to discuss the investigation and spoke on condition of anonymity.

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 center for the disabled, where Farook's co-workers had gathered for a holiday party Wednesday. Farook had attended the event but slipped out and returned in battle dress.

Four hours later and two miles away, the couple died in a furious gun battle in which they fired 76 rounds, while 23 law officers unleashed about 380, police said.

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

At the social service center, the couple left three rigged-together pipe bombs with a remote-control detonating device that apparently malfunctioned, and they had more than 1,600 rounds of ammunition remaining when police killed them in their rented SUV, Burguan said.

At a family home in the nearby town of Redlands, they had 12 pipe bombs, tools for making more, and more than 3,000 additional rounds of ammunition, the chief said.

"We don't know if this was workplace rage or something larger or a combination of both," Attorney General Loretta Lynch said in Washington, echoing President Barack Obama. "We don't know the motivation."

Investigators are trying to determine whether Farook, who was Muslim, became radicalized- and, if so, how - as well as whether he was in contact with any foreign terrorist organization, said the US intelligence official, who was not authorized to discuss the matter publicly and spoke on condition of anonymity.