(CNN) -- A district attorney has decided against charges in the case of a police officer who fatally shot a mentally ill man more than a dozen times in Milwaukee.
The officer, Christopher Manney, killed Dontre Hamilton during a confrontation in April.
The officer says he opened fire when Hamilton grabbed his baton and struck him with it.
Manney has since been fired for not following protocols, but he will not be charged.
"I've come to the conclusion that criminal charges are not appropriate in this case, and I am releasing all of the information related to this investigation so that you, the public, can see all the facts related to this decision," Milwaukee County District Attorney John Chisholm told reporters Monday.
In a report, he wrote that the officer's use of force was "justified self-defense and that defense cannot be reasonably overcome to establish a basis to charge Officer Manney with a crime."
Chisholm anticipated that some might be upset with his decision and, in fact, protesters began taking to the streets Monday night.
According to local media, the officer is white; Hamilton was black.
"On a human level, of course it's tragic. Anytime I have to tell a family that I can't bring justice to them when one of their loved ones has died, it's always tragic. It's terrible," Chisholm said.
"The reason that our job is unique is our obligation is not to tell people necessarily what they want to hear. We have to follow our ethical obligations and the law, and sometimes that's very difficult ... But it's a privilege to be able to do the job, and we're committed to doing it the right way," he told reporters.
Following Chisholm's announcement, the U.S. Department of Justice said that it would conduct a federal civil rights review of the case, which comes amid ongoing national protests around race and law enforcement in America.
Earlier this year in Ferguson, Missouri, a jury decided not to indict Darren Wilson, a white officer who shot Michael Brown, an unarmed black teenager.
Charges were also not filed in the death of Eric Garner, a black man, who died in Staten Island, New York, after an officer put him in a chokehold.