An American jury acquitted teenager Kyle Rittenhouse on Friday of murder in the fatal shooting of two men during racial justice protests in a decision that re-ignited fierce debate about gun rights and the boundaries of self-defence in the United States.

Jurors found Rittenhouse, 18, not guilty on all charges: two counts of homicide, one count of attempted homicide for wounding a third man, and two counts of recklessly endangering safety in protests marred by arson, rioting and looting on August 25, 2020, in Kenosha, Wisconsin.

Rittenhouse broke down sobbing after the verdict and collapsed to the floor before being helped back into his chair, his hands shaking. His mother also wept.

Amid a heavy law enforcement presence, several dozen protesters lined the steps outside the courthouse after the verdict was read, some carrying placards in support of Rittenhouse and others expressing disappointment. By early evening, the crowd had thinned to a handful of people and there was no sign of disturbances in the city.

"We are all so very happy that Kyle can live his life as a free and innocent man, but in this whole situation there are no winners, there are two people who lost their lives and that's not lost on us at all," David Hancock, a spokesperson for the Rittenhouse family, told Reuters.

Rittenhouse shot and killed Joseph Rosenbaum, 36, and Anthony Huber, 26, and fired a bullet that tore a chunk off the arm of Gaige Grosskreutz, 28. Rittenhouse claimed self-defence.

US President Joe Biden, who during last year's election campaign tweeted a video that appeared to link Rittenhouse to white supremacists, said on Friday he supported the jury's decision and urged Americans to react with calm.

"While the verdict in Kenosha will leave many Americans feeling angry and concerned, myself included, we must acknowledge that the jury has spoken," Biden said.

Elsewhere reactions showed the country's deep partisan divisions. The verdict was greeted with outrage by many on the political left and celebrated by gun-rights supporters.

"It is unconscionable our justice system would allow an armed vigilante ... to go free," the Congressional Black Caucus said in a statement.

The thorny issue of race also hung over the case, although Rittenhouse and the men he shot were all white. Some Black activists said on Friday the US police and courts would have treated the teenager more harshly if he had been Black.

But conservatives saw the verdict as a validation of the US Constitution's Second Amendment, which grants Americans the right to bear arms.

US congressman Madison Cawthorn, a Republican representative from North Carolina, said on Instagram: "Kyle Rittenhouse is not guilty my friends. You have a right to defend yourselves. Be armed, be dangerous and be moral."

Repeatedly attacked

In reaching its verdict after more than three days of deliberations, the jury contended with duelling narratives from the defence and prosecution that offered vastly different portrayals of the teenager's actions on the night of the shootings.

The defence argued that Rittenhouse had been repeatedly attacked and had shot the men in fear for his life. They said he was a civic-minded teen who carried a medical kit in addition to his gun and was in Kenosha to protect private property after several nights of unrest in the city south of Milwaukee.

The violence followed the police shooting of a Black man named Jacob Blake, who was left paralysed from the waist down.

The prosecution portrayed Rittenhouse as a reckless vigilante who provoked the violent encounters and showed no remorse for the men he shot with his AR-15-style rifle.

Wisconsin criminal defence lawyer Daniel Adams, who closely followed the trial, called the verdict “very dramatic but not entirely surprising”.

Most lawyers "who looked at the evidence had a feeling the state would not be able to clear the threshold of disproving self-defence beyond a reasonable doubt”, he said.

Live-streamed and dissected by cable TV pundits daily, the trial unfolded during a time of social and political polarisation in the United States. Gun rights are cherished by many Americans and are enshrined in the US Constitution even as the nation experiences a high rate of gun violence and the easy availability of firearms.

Rittenhouse, who testified that he had no choice but to open fire to protect himself, is viewed as heroic by some pro-gun conservatives who consider the shootings justified. Many on the left view Rittenhouse as a vigilante and an embodiment of out-of-control American gun culture.

Protests against racism and police brutality turned violent in many US cities after the police killing of Black man George Floyd in Minneapolis three months before the Kenosha shootings.

The Rittenhouse verdict ended the highest-profile US civilian self-defence case since a man named George Zimmerman was acquitted in the fatal shooting of Trayvon Martin, an unarmed Black teenager, in Florida in 2013.

With so much of that night in Kenosha caught on cellphone and surveillance video, few basic facts were in dispute. The trial instead focused on whether Rittenhouse acted reasonably to prevent "imminent death or great bodily harm", the requirement for using deadly force under Wisconsin law.

The prosecution, led by Kenosha County Assistant District Attorney Thomas Binger, sought to paint Rittenhouse as the aggressor and repeatedly stressed that he was the only one to kill anyone that night.

Riskey decision

Rittenhouse's gun was loaded with 30 rounds of full metal jacket bullets, which are designed to penetrate their target. The jury saw a series of graphic videos, including the moments after Rittenhouse fired four rounds into Rosenbaum, who lay motionless, bleeding and groaning. Another video showed Grosskreutz screaming, with blood gushing from his arm.

Rittenhouse testified in his own defence last Wednesday in the trial's most dramatic moment — a risky decision by his lawyers given his youth and the prospect of tough prosecution cross-examination. Rittenhouse broke down sobbing at one point and emphasised that he acted out of fear for his life.

"I did what I had to do to stop the person who was attacking me," he said.

His defence counsel, Mark Richards, said Rittenhouse has difficulty sleeping at night and was being treated for post-traumatic stress disorder. He said the defence team decided to have him testify after they tested two versions of their case before mock juries, one with him testifying and one without.

"It was substantially better when we put him on," Richards told reporters after the verdict. "In Wisconsin, if you don't put a client on the stand, you're going to lose. Period."

Rittenhouse testified that he shot Huber after he had struck him with a skateboard and pulled on his weapon. He said he fired on Grosskreutz after the man pointed the pistol he was carrying at him — an assertion Grosskreutz acknowledged under questioning from the defence. Rittenhouse testified that he shot Rosenbaum after the man chased him and grabbed his gun.

Huber's parents, Karen Bloom and John Huber, said in a statement they were "heartbroken" by the verdict.

"It sends the unacceptable message that armed civilians can show up in any town, incite violence, and then use the danger they have created to justify shooting people in the street."