Milwaukee County District Attorney John Chisholm said the fatal shooting of Dontre Hamilton in Red Arrow Park was "justified self-defense."
A 10 a.m. news conference will be held, further explaining the decision.
Chisholm called for a special investigation into whether charges should be filed against former Milwaukee police officer Christopher Manney in the April shooting death of Hamilton.
Manney, 38, approached Hamilton after receiving a complaint that he was sleeping in Red Arrow Park.
Police Chief Ed Flynn said Manney approached Hamilton, identified himself and asked Hamilton to stand up. When Hamilton stood up, Manney approached Hamilton from behind, reached under Hamilton’s arms, placed his hands on his chest to conduct a pat-down search and asked if Hamilton had any weapons.
As Manney began the search, Hamilton began fighting with Manney.
Flynn said Manney tried to use his baton to subdue Hamilton, but Hamilton was able to take the baton from Manney. Hamilton swung the baton at Manney multiple times and struck Manney on the side of the neck with the baton. Manney said he "was violently attacked with fists and a weapon to my head."
Fearing that Hamilton would seriously injure or kill him, Manney used his service weapon and shot Hamilton.
"This was a tragic incident for the Hamilton family and for the community," Chisholm said. "Based on all the evidence and analysis presented in this report, I come to the conclusion that Officer Manney'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."
In a Monday news conference, Chisholm said that all forensic, legal and expert evidence leads him to believe that officer Manney shot in self-defense.
"The ACLU of Wisconsin regrets District Attorney Chisholm’s decision because it leaves a cloud of uncertainty over the circumstances of and the responsibility for Mr. Hamilton’s death," said executive director Chris Ahmuty. "If officer Christopher Manney did not violate the law, then is anyone legally responsible for Mr. Hamilton's death?"
Hamilton family lawyer Jonathan Saffron released a statement following the district attorney's decision:
"The Dontre Hamilton shooting death case has now resulted in the Milwaukee County District Attorney adding yet another decision to the list of cases around the country with no criminal charges being filed against a police officer who shot and killed an African American male citizen," he said. "This is a case which cries out for justice, criminal charges against Christopher Manney, and accountability to Dontre Hamilton’s family."
In October, Flynn fired Manney for failing to follow proper protocols in the situation.
Flynn said Manney accurately determined Hamilton was an "emotionally disturbed person, but treated him as a dangerous criminal instead of following his training and treating Mr. Hamilton as an EDP."
"It was an incorrect, wrong decision that placed him ultimately in legitimate deadly jeopardy," Flynn said.
Mike Crivello, president of the Milwaukee Police Association, said, "The use of deadly force by an officer is a difficult decision that must be made accurately and instantaneously. The MPA is confident in the officer's training and trust the action taken was of necessity."
Manney worked in the Milwaukee police operations division and had been on the force for 13 years.
The Wisconsin National Guard has called up members to respond to potential protests in the wake of the announcement.