Source: BBCNews
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 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.