Police found thousands of rounds of ammunition for multiple types of guns at the scene. Photo: AP

Bomb equipment, weapons and thousands of rounds of ammunition were found by police in a raid after a shootout that killed the two suspects.

Authorities still have not found a motive in the attack by Syed Rizwan Farook, 28, and Tashfeen Malik, 27.

Police said the attack indicated there had been "some degree of planning".

San Bernardino Police Chief Jarrod Burguan said it appeared that the duo was prepared to carry out another attack.

"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.

In the shootout with police hours after the attack, Farook and Malik fired 76 rounds of ammunition at the officers and the officers fired 380 rounds back.

Two police officers were injured during the pursuit.

It marks the deadliest mass shooting in the US since 26 people were killed at a school in Newtown, Connecticut in 2012.

Authorities are expected to start releasing the names of the victims on Thursday.

President Barack Obama said the FBI had taken over the investigation from local authorities.

"It is possible that this was terrorist-related, but we don't know. It's also possible that this was workplace-related," Mr Obama said.

The FBI cautioned that authorities needed time to investigate. But many Republican presidential candidates such as New Jersey Governor Chris Christie were convinced it was terrorism.

Former Homeland Security Secretary Tom Ridge told the BBC said the information already made public points to terrorism.

"I would characterise this as a terrorist attack," he said. "Workplace attacks are a lot more spontaneous, they're not planned their not premeditated, they normal involve one person, they never involve pipe bombs."

Law enforcement officials are also looking at possible "co-conspirators" and have been in touch with the suspects' relatives.

Farook, who was born in the US, was not "on the radar" of law enforcement, Mr Burguan said.

Police riddled a black SUV - identified earlier as the getaway car - with bullets, killing two suspects

He worked for the past five years as a restaurant inspector for the San Bernardino County Health Department. The Los Angeles Times reported that co-workers said he was a devout Muslim but rarely discussed his religion.

He and Malik were recently married and had a six-month-old baby, who they dropped off at its grandmother's house before the attack.

On Wednesday, Farook and his colleagues were attending a holiday party at the Inland Regional Center.

Co-worker Patrick Baccari said he was sitting at the same table as Farook before he abruptly left, leaving his coat.

Wearing military-style clothing and armed with assault rifles and semi-automatic handguns, Farook returned with Malik a short time later to open fire.

When the shooting started, Mr Baccari took refuge in a bathroom and was hurt when shrapnel sliced through the wall.

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