The shooting this week at an office party in California that left 14 people dead is now being investigated as an "act of terrorism," the US Federal Bureau of Investigation said Friday.

The shooting this week at an office party in California that left 14 people dead is now being investigated as an "act of terrorism," the US Federal Bureau of Investigation said Friday.

"We have uncovered evidence that has led us to learn of extensive planning," David Bowdich, the assistant director in charge of the FBI field office in Los Angeles, told reporters.

The female assailant in California's mass shooting had pledged allegiance to the head of the ISIL group on Facebook, reports said Friday, as the attackers' home was thrown open to the public.