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The gunfire erupted late on Tuesday, just before midnight, during the third straight night of unrest in Kenosha over the police shooting of a Black man, Jacob Blake.
Sheriff David Beth said one victim was shot in the head and the other in the chest, the Milwaukee Journal Sentine l reported. A third person was also shot, but the wounds were not believed to life-threatening.
Beth said that investigators had reviewed footage and that he was confident a man would be arrested soon.
According to witness accounts and video footage, police apparently let the young man responsible for some or all of the shootings walk past them with a rifle over his shoulder as members of the crowd were yelling for him to be arrested because he had shot people.
The sheriff told the Journal Sentinel that armed people had been patrolling the city's streets in recent nights, but he did not know if the gunman was among them.
"They're a militia," Beth said. "They're like a vigilante group."
The FBI said it is assisting in the case.
Wisconsin Lt. Gov. Mandela Barnes, who is Black, said in an interview with the news program "Democracy Now!" that the shootings were not surprising and that white militias have been ignored for too long.
"How many times across this country do you see armed gunmen, protesting, walking into state Capitols, and everybody just thinks it's OK?" Barnes said. "People treat that like it's some kind of normal activity that people are walking around with assault rifles."
Cellphone video of at least two of the shootings that was posted online shows a young man with a rifle jogging down the middle of a street as a crowd and some police officers follow him. Someone in the crowd can be heard asking, "What did he do?" and another person responds that the man had shot someone.
The man with the gun stumbles and falls, and as he is approached by people in the crowd, he fires three or four shots from a seated position, hitting at least two people, including one who falls over and another who stumbles away to cries of "Medic! Medic!"
A witness, Julio Rosas, 24, said that when the gunman stumbled and fell, "two people jumped onto him and there was a struggle for control of his rifle. At that point during the struggle, he just began to fire multiple rounds and that dispersed people near him."
"The rifle was being jerked around in all directions while it was being fired," Rosas said.
In the cellphone footage, as the crowd scatters, the gunman stands up and continues walking down the street as police cars arrive. The man puts up his hands and walks toward the squad cars, with someone in the crowd yelling at police that the man had just shot someone, but several of the cars drive past him toward the people who had been shot.
Protester Devin Scott told the Chicago Tribune that he witnessed one of the shootings.
"We were all chanting 'Black lives matter' at the gas station and then we heard, boom, boom, and I told my friend, ''That's not fireworks,'" said Scott, 19. "And then this guy with this huge gun runs by us in the middle of the street and people are yelling, 'He shot someone! He shot someone!' And everyone is trying to fight the guy, chasing him and then he started shooting again."
Scott said he cradled a lifeless victim in his arms, and a woman started performing CPR, but "I don't think he made it."
In other widely circulating video, police can be heard thanking and tossing bottled water from an armoured vehicle to what appear to be armed civilians walking the streets.
"We appreciate you being here," an officer is heard saying over a loudspeaker.
At a news conference earlier on Tuesday, Ben Crump, the lawyer for Blake's family, said that Blake was shot multiple times by police on Sunday and that it would "take a miracle" for him to walk again. He called for the officer who opened fire to be arrested and for the others involved to lose their jobs.
Blake was shot, apparently in the back, as he leaned into his SUV, three of his children seated inside. The shooting was captured on cellphone video and ignited new protests in the US three months after the death of George Floyd under the knee of a Minneapolis police officer touched off a nationwide reckoning over racial injustice.
Kenosha police have said little about what happened other than that they were responding to a domestic dispute. They have not said why the officers opened fire or whether Blake was armed, and they have not disclosed the race of three officers on the scene.
The 29-year-old Blake underwent surgery on Tuesday, according to Crump, who added that the bullets severed his spinal cord and shattered his vertebrae. Another attorney said there was also severe damage to organs.
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