California gunman Syed Rizwan Farook had been in contact with known Islamic extremists on social media, a US intelligence official has said, 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 left 14 people dead was terrorism, a workplace grudge or some combination.

The husband-and-wife killers were not under FBI scrutiny before the massacre, said a second US official, who likewise was not authorised 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 centre 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 gunbattle in which they fired 76 rounds, while 23 law officers unleashed about 380, police said.

Yesterday, 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 over 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 radicalised and, if so, how as well as whether he was in contact with any foreign terrorist organisation, said the US intelligence official, who was not authorized to discuss the matter publicly and spoke on condition of anonymity.

The same official said Farook had been in touch on social media with extremists who were under FBI scrutiny.

The second US official said the FBI was treating the attack as a potential act of terror but had reached no conclusion that it was. The official said Farook's contacts online did not involve any significant players on the agency's radar and dated back some time, and there was no immediate indication of any surge in communication ahead of the shooting.