Madison, Wisconsin: A Wisconsin police officer who fatally shot an unarmed biracial teenager in March, prompting several days of peaceful protests, will not be charged, a prosecutor said on Tuesday.

Officer Matt Kenny used justified lethal force in the March 6 shooting of Tony Robinson, 19, who struck the 12-year police veteran in the head, Dane County District Attorney Ismael Ozanne said.

‘I conclude that this tragic and unfortunate death was the result of a lawful use of deadly police force and that no charges should be brought against officer Kenny,’ Ozanne said in a 25-minute statement, during which he repeatedly mopped his face.

The shooting in Madison, Wisconsin's capital, was one of a number of officer-involved deaths that have led to increased scrutiny of police use of force in the United States, particularly against young black men.

Ozanne said he was sorry for Robinson's death and that he shares concerns of many Americans about racial profiling by law enforcement officers. He noted that he is biracial like Robinson and also fears being singled out by police because of the color of his skin.

Kenny's attorney said the decision was appropriate, while Robinson's family members expressed disappointment.

Several hundred demonstrators were marching from the gray house where Robinson was shot toward the state Capitol Building, singing gospel songs behind a street-wide ‘Black Lives Matter’ banner. A protest is also planned on Wednesday morning.

Police warned that there had been credible threats against officers if the district attorney decided not to bring charges.

Ozanne said Kenny was responding to multiple emergency calls reporting that a man had battered someone and was dodging traffic in the street. Robinson's friends called 911 to say they were afraid of him because he was acting violently and was on drugs.

Ozanne said Kenny followed Robinson into a dwelling and shot him seven times after the teen struck Kenny in the head. The teenager had psilocybin mushrooms, marijuana and the psychoactive drug Xanax in his system.

The prosecutor's announcement came days after the US Justice Department announced a civil rights investigation into the Baltimore police department's use of force to determine if there are patterns of discriminatory policing.

Riots broke out in the streets of Baltimore over the 19 April death of Freddie Gray, 25, who died after suffering a spinal injury while in police custody. Baltimore's chief prosecutor has charged six police officers in Gray's death.

There were large but orderly demonstrations in Madison after Robinson's shooting. The city of 240,000 people is nearly four-fifths white and 7 per cent African-American, according to U.S. Census figures.

Last year, Robinson pleaded guilty to armed robbery and was put on probation. Sentencing documents show it was his first brush with the law, and he was not the armed person in the group that committed the robbery.

Kenny has been on paid administrative leave during the investigation. In 2007, he was involved in a fatal shooting that was found to be justified.