Across the border in Wisconsin, Gov. Scott Walker has activated the Wisconsin National Guard to help law enforcement if needed.
There were scenes of unrest in Milwaukee overnight after a man fleeing police after a traffic stop was shot and killed.
The Associated Press reports that police say the 23-year-old man was armed, but it's not clear if he was pointing the gun or aiming at officers.
According to the Milwaukee Journal Sentinel, the killing sparked an hours-long standoff between an "angry crowd" and police, which turned violent at around 10:15 p.m. when a gas station was set on fire and gunshots were heard.
Later that night, fires were started at businesses including a BMO Harris Bank, a beauty suppliers, and an O'Reilly Auto Parts store, with Mayor Tom Barrett appealing for calm during a midnight press conference.
On its Twitter account, Milwaukee police said there were also instances of rocks being thrown at police, squad cars set on fire, and officer requiring hospital treatment after he was hit in the head by a brick thrown through a squad car window.
NBC News reports that at least three people had been arrested as of early Sunday morning, and while the crowd had thinned since Saturday night, a Milwaukee police spokesman said there are still "a lot of people out."
Milwaukee PD tweeted Sunday morning that it is restoring order to the Sherman and Burleigh neighborhoods where the unrest unfolded, and deployments were being reduced.
According to a police statement, the 23-year-old killed by police had a lengthy arrested record and was armed with a semi-automatic handgun, which was reported stolen during a burglary in Waukesha this past March.
Police said 500 rounds of ammunition were also stolen with the handgun.
The officer involved, a 24-year-old male, has been with MPD for six years – three as an officer – and will be placed on administrative leave during the investigation.
Walker said in a statement that he "will not comment on the specifics of the case as it is now under this investigation." However, he hopes "people will give law enforcement the respect that they deserve for working so hard to keep us safe."
The unrest comes amid a backdrop of growing discontent about fatal police shootings of black Americans.