US Department of Homeland Security officials have categorised the killing of a federal contract security officers as an "act of terrorism".
Their identities have not been released and the suspect has not been named; officials have not clarified whether the shooting was related to a massive protest in Oakland following the police killing of George Floyd in Minneapolis.
Acting DHS secretary Chad Wolf called the alleged shooter an "assassin" who "cowardly shot" the officers "as they stood watch over a protest".
He said the injured officer is in "critical condition fighting for his life".
During a press conference on Saturday, DHS Acting Deputy Secretary Ken Cuccinelli claimed that "there are currently threats by some to attack police stations and federal buildings".
"That violence will not be tolerated," he said. "We're also committed to ensuring that it won't succeed anywhere, and let me be clear, when someone targets a police officer or a police station with an intention to do harm and intimidate — that is an act of domestic terrorism."
FBI officials reported that "an individual inside the vehicle began firing gunshots at contract security officers for the Federal Protective Service of the Department of Homeland Security" as hundreds of protesters were demonstrating in Oakland.
The Federal Protective Service is used "to prevent, protect, respond to and recover from terrorism, criminal acts, and other hazards" that threat "critical infrastructure, services, and the people who provide or receive them," according to DHS.
Twenty-two people were arrested during the protests.