US Police Chief Jarrod Burguan said the two suspects in the California shooting were armed with assault rifles and handguns and dressed in "assault-style" clothing.
The police chief said he knew of no possible motive for the attack.
David Bowdich, an assistant regional FBI director, said authorities had not yet ruled out whether the shooting was an act of terrorism.
He told reporters: "It is a possibility, but we don't know that".
"It's possible it goes down that road. It's possible it does not."
Burguan said earlier: "Obviously, at a minimum, we have a domestic-type, terrorist-type situation that occurred here."
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