Officials said they received a tip about the vehicles. When officers found them — a bread truck, black school bus and minivan — they followed the occupants, a police news release said.
The department did not specify which states the license plates were from. While under police surveillance, the group stopped at a gas station and attempted to fill up multiple fuel cans, the statement said.
"Suspecting that the occupants of these vehicles were preparing for criminal activity related to the civil unrest, officers attempted to make contact and investigate," police said.
The people inside the bread truck and school bus were detained. The minivan drove away but was stopped a short time later, police said. Officers forced their way into the vehicle and arrested those inside, police said.
Inside the vehicles, officers found fireworks, helmets, gas maks, protective vests and suspected controlled substances. The nine people were arrested on suspicion of disorderly conduct.
Calls to the police department from Fox News were not returned.
Like protests in other cities, Kenosha officials have suspected some engaged in looting and other criminal activity are not locals. The unrest began Sunday after Kenosha Police officer Rusten Sheske shot Jacob Blake seven times in the back hours earlier.
A protester lights a cigarette on a garbage truck that was set on fire during protests late Aug. 24, in Kenosha, Wis., sparked by the shooting of Jacob Blake by a Kenosha Police officer a day earlier. (AP Photo/Morry Gash)
"A huge part of me thinks that a lot of our issues start when different people with different agendas come here to Kenosha," Kenosha County Sheriff David Beth said Thursday.
Kyle Rittenhouse, 17, came from Illinois armed with a rifle and fatally shot two people during Tuesday's protests, authorities said. He is charged with first-degree intentional homicide and is awaiting extradition from Illinois, where he was arrested, to Wisconsin.