Police have found bomb equipment, weapons and thousands of rounds of ammunition at San Bernardino shooters’ home.
The attackers killed 14 people and wounded 21 at San Bernardino’s Inland Regional Center in California.
Authorities still have not found a motive in the attack by Syed Rizwan Farook, 28, and Tashfeen Malik, 27.
According to police, the attack indicated there had been “some degree of planning”.
“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, Syed Rizwan Farook and Tashfeen Malik fired 76 rounds of ammunition at the officers and the officers fired 380 rounds back.
Two police officers were injured during the pursuit.
San Bernardino attack 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 December 3.
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.
Syed Rizwan Farook, who was born in the US, was not “on the radar” of law enforcement, Jarrod 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 Tashfeen Malik were recently married and had a six-month-old baby, who they dropped off at its grandmother’s house before the attack.
On December 2, Syed Rizwan 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 Syed Farook before he abruptly left, leaving his coat.
Wearing military-style clothing and armed with assault rifles and semi-automatic handguns, Syed Farook returned with Tashfeen Malik a short time later to open fire.
When the shooting started, Patrick Baccari took refuge in a bathroom and was hurt when shrapnel sliced through the wall.