Kyle Rittenhouse, a teenager who shot dead two people during Wisconsin racial unrest last year, was cleared of all charges.
The 18-year-old argued he was acting in self-defense when he shot the men and injured a third in Wisconsin.
The verdict on November 19, which followed a high-profile trial that divided the US, sparked protests in some cities.
However, politicians, including President Joe Biden, and families of the victims have urged restraint.
“What we need right now is justice, not more violence,” lawyers for the family of Gaige Grosskreutz, who was shot and injured by Kyle Rittenhouse, said in a statement.
“I urge everyone to express their views peacefully, consistent with the rule of law,” President Joe Biden told reporters outside the White House.
Despite the calls for calm, a riot was declared by police in the city of Portland, Oregon, on Friday evening as some 200 people broke windows and threw objects.
There were also protests in Chicago and New York, but they were relatively low-key compared to the widespread civil unrest that the US had seen previously.
Kyle Rittenhouse said he was acting in self-defense when he fatally shot Joseph Rosenbaum, 36, and Anthony Huber, 26, and injured Gaige Grosskreutz, 28, in August 2020 in the city of Kenosha, Wisconsin.
The teenager and the men he shot are all white. The incident happened during violent protests over the shooting of a black man, Jacob Blake, by a white police officer.
Jacob Blake’s uncle was outside the court when Kyle Rittenhouse was acquitted.
“We’re going to continue to fight and we’re going to continue to be peaceful. Let freedom ring,” Justin Blake said.
Anthony Huber’s family said the verdict “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.”
Kyle Rittenhouse is now in an undisclosed location, a spokesman for his family told CBS.
“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,” they said.
The verdict has prompted strong reactions from both sides.
Derrick Johnson, the president of the civil rights organization NAACP, tweeted that the verdict “is a reminder of the treacherous role that white supremacy and privilege play within our justice system”.
Prominent civil rights activist Reverend Al Sharpton said the “outrageous and dangerous” verdict will encourage vigilantes to use violence to assert their power.
“These continue to be dark days for black people killed at the hands of people that believe our lives do not matter,” he said in a statement.
There are also concerns about the legal precedence the verdict could set for future cases.
California’s Governor Gavin Newsom wrote: “America today: you can break the law, carry around weapons built for a military, shoot and kill people, and get away with it.”
President Biden said he was “angry and concerned” at the verdict, but that “the jury has spoken”.
Conservatives argue Kyle Rittenhouse asserted his right to bear arms and defended himself.
Former President Donald Trump congratulated him in a statement emailed to his supporters.
“If that’s not self-defense, nothing is!” he wrote.
Several Republican lawmakers said they would like to offer a congressional internship to Kyle Rittenhouse.
One of them was the representative of North Carolina, Madison Cawthorn, who on an Instagram story said: “You have a right to defend yourself, be armed, be dangerous and be moral.”
Following the verdict, right-wing Fox News host Tucker Carlson posted a clip of an exclusive interview with the teenager, to be broadcast on November 22.