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President Donald Trump says federal law enforcement will be sent to Kenosha, Wisconsin, following unrest sparked by the police shooting of a black man.
Jacob Blake, 29, was shot and injured by police on Sunday as he leaned into his car as his children watched.
Protests have been ongoing for several nights, and on Tuesday, two people were shot dead and another was injured. A teenager has been charged with murder.
Mr Blake is recovering in hospital and is conscious, his family say.
But his lawyers say it will take “a miracle” for him to walk again.
The protests over Mr Blake’s shooting have at times have turned violent, causing damage to property.
The protests over Mr Blake’s shooting have at times have turned violent, causing damage to property.
Tuesday night’s incident was believed to have involved protesters against Mr Blake’s shooting and armed men guarding a petrol station.
Hours later, police in neighbouring Illinois said they arrested a 17-year-old on suspicion of first-degree murder.
Announcing his move in a tweet on Wednesday, Mr Trump said: “We will NOT stand for looting, arson, violence, and lawlessness on American streets. My team just got off the phone with Governor Evers who agreed to accept federal assistance (Portland should do the same!).”
Shortly following Mr Trump’s tweet, Wisconsin Governor Tony Evers said he had authorised 500 National Guard troops to support law enforcement efforts.
What happened during the protests?
Hours after Mr Blake was shot on Sunday, hundreds rallied outside Kenosha’s police headquarters. As the protests went on, cars were set alight, armed robberies were reported and a night-time curfew was eventually put in place.
National Guard troops were deployed on Monday. Protesters said police and troops used tear gas, rubber bullets and smoke bombs.
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On Tuesday, hundreds of protesters marched through the city. A small group threw fireworks and water bottles at police, who responded with rubber bullets and tear gas.
A spokesman for the Wisconsin Professional Police Association told the BBC individuals and groups were exercising their own form of vigilantism because law enforcement was spread very thin across the city.
On social media, footage showed a man with a rifle being chased by a crowd before he fell to the ground and appeared to fire multiple rounds at them. Other video shows armed civilians, many dressed in military fatigues, congregating outside businesses they said they were protecting.