The shooters wearing "assault-style clothing" targeted the party at the Inland Regional Centre in San Bernardino and fled after the carnage in a black SUV, leading to a lengthy police chase. Hours later, the attackers were killed by police in an encounter.
Jarrod Burguan of the San Bernardino police identified the two suspects killed in the shootout with police as Syed Rizwan Farook, 28, and Tashfeen Malik, 27, both of Pakistani-origin.
Farook and Malik were married and the parents of a 6- month-old daughter, according to Farook's brother-in-law Farhan Khan.
Farook was born in the US and was an American citizen. He was an environmental specialist with the county health department who sometimes worked at the Inland Regional Center.
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Farook had been at the party and left "angry" amid a dispute, Burguan said.
He and his wife then returned with explosive devices, assault-style rifles and handguns. They were dressed in "dark, kind of tactical gear", the police chief said.
"Based upon how they were equipped, there had to be some degree of planning that went into this. I don't think they just ran home and put on these tactical clothes, grabbed guns and came back on a spur of the moment thing," he said.
At least 14 people died in the shooting, police said, describing it as an incident of "mass shooting".
Seventeen others were injured - mostly by bullets from automatic-style rifles. Some persons injured in the panic to escape.
"A device that looked like a bomb also flew out of the vehicle," an official was quoted as saying by media reports.
The attack was the deadliest shooting on US soil since the Sandy Hook school massacre in 2012.