The attackers who killed 14 people and wounded 21 at a social services centre in California had an arsenal of weaponry in their home, police said.
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, 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," he said.
"We're going to have to search ourselves as a society to make sure we can take basic steps that would make it harder - not impossible, but harder - for individuals to get access to weapons."
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.

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.

BBC