A shooting at a social services centre in California on Wednesday had "a degree of planning" behind it, San Bernardino police have said.

Bomb equipment, weapons and thousands of rounds of ammunition were found by police in a search following a shootout that killed at least 14 people, as well as the two suspects.

"Nobody just gets upset at a party, goes home and puts together that kind of an elaborate scheme or plan to come back and do that", police chief Jarrod Burguan said.