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The Madison, Wisconsin, cop who shot and killed unarmed teenager Tony Terrell Robinson Jr. won't face charges, the prosecutor said Tuesday.

Dane County District Attorney Ismael Ozanne called the shooting of Robinson, 19, on March 6 by Officer Matt Kenny a "lawful use of deadly police force," saying that Robinson had Xanax, mushrooms and marijuana in his system and that multiple witnesses told investigators Robinson was "tweaking out" on mushrooms, jumping in front of cars and intimidating bystanders.

Ozanne, the first African-American district attorney in Wisconsin's history, called Robinson's death "tragic and unfortunate," saying, "I am cognizant of the very real racial disparities and inequalities that exist in this county."

"My decision is not based on emotion,” he said. "This decision is guided by the rule of law and the oath I took to uphold the constitution."

But Sharon Murray, Robinson's grandmother, said: "This is politics and not justice." She called her grandson "a great kid" who had been "set up."

Jon Loevy, the family's attorney, many questions about the shooting remained unanswered — specifically about differences in Kenny's accounts of where he was when he fired the last of seven shots.

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"Apparently, the officer suffered some kind of amnesia," Loevy said.

Kenny, 45, shot Robinson seven times after the two got into a confrontation when Kenny was responding to reports of a battery at a residence, Ozanne and police said. An autopsy found that Robinson was shot in the head and torso.

Ozanne quoted extensively from a 27-page report from the state Division of Criminal Investigation, the agency in charge of the investigation. The report said Kenny described being repeatedly punched and charged by Robinson, who he feared would either kill him or knock him out and take his weapon.

"Officer Kenny stated that he did not have the ability to use non-lethal force during this incident, based on space and time considerations," according to the report.

A friend told investigators that he had been getting texts from other friends saying Robinson was "going crazy" and punching the walls of the apartment, the report said.

But Robinson's mother state investigators that her son was a "gentle giant, and only wanted to belong," according to the report. She called him "a good boy who did not deserve this."

The Wisconsin Professional Police Association, the state police union, called Ozanne's decision "appropriate," adding: "As a city, we must now come together to engage in a community-wide dialogue to strengthen the relationship between law enforcement and the people it serves."

The American Civil Liberties Union of Wisconsin said it "regrets" Ozanne's conclusion, saying it "leaves a cloud of uncertainty" over Robinson's death.

"Are police officers above the law?" it asked in a statement.

Ozanne's office released hundreds of pages of reports detailing Kenny's and Robinson's history at the end of March. Kenny also fatally shot a suspect in 2007, according to the documentation and police reports at that time. He was cleared of wrongdoing.

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Reporters are packed into the news conference where the DA is set to make an announcement about #TonyRobinson pic.twitter.com/akckbtvPBj — Mary Spicuzza (@MSpicuzzaMJS) May 12, 2015