OAKLAND, Calif. (KTVU) - Video shows a crowd cheering as protesters broke into the Alameda County Superior Courthouse near Oakland's Lake Merritt and set it ablaze on Saturday night.
"An accelerant. whether it was gasoline or some other flammable material, was put in there, and it looked like a firework went in there and it ignited that, creating the combustion that looks like an explosion," said Alameda County Sheriff's Sgt. Ray Kelly.
Original doors and windows from the 1930s were vandalized. Protesters also sprayed graffiti on the courthouse, causing about $200,000 in damage.
"It's a very old building, and there's a lot of history there, but also it's a working courthouse," Kelly said.
Protesters also smashed windows at businesses near City Hall. And at the Oakland Police Department, so-called "agitators" set off fireworks, fired paint guns and shined lasers at police and law-enforcement aircraft, police said.
Oakland police made four arrests. Two suspects are from Oakland, a third from Hayward and the fourth from Humboldt County.
Oakland officials fear the images from Saturday night's protests will lead President Trump to make good on his threat to send federal officers to the city.
In Portland, federal agents in camouflage have been detaining protesters without explanation.
"We don't need to play into Donald Trump's political games," said Oakland City Councilmember Dan Kalb.
Kalb said federal officers aren't welcome in Oakland.
"You are not invited here, we do not want you to come here, and we will do what we can within the law to prevent you from coming here," Kalb said.
Kalb said the council is set to pass a resolution Tuesday that directs city officials to "take any and all lawful necessary steps" to protect against any harm caused by federal officers.
The councilmember declined to elaborate.
"That's a legal strategy, and you don't kind of telegraph that in advance, right?" Kalb said.
Oakland Mayor Libby Schaaf, whose own home was vandalized last week, is making a distinction between peaceful protesters and others out to break the law.
In a statement from over the weekend, Schaaf said, "We celebrate passionate protest, but Oaklanders need to know that when they attend protests after dark, they may be providing cover for agitators who are more intent on stoking civil unrest than advancing racial justice."