A white Milwaukee police officer fired after killing a mentally ill black man in April won't face criminal charges, the top county prosecutor said Monday, a decision that prompted the U.S. attorney to later announce a federal investigation of the incident.

Milwaukee County District Attorney John Chisholm said Christopher Manney won't be charged because he shot Dontre Hamilton in self-defense. Manney is at least the third white police officer across the country to avoid charges in the past month after a confrontation that led to a black man's death.

"(Manney's) use of force was privileged and justified," Milwaukee County District Attorney John Chisholm said during a news conference. "On a human level, of course, it's tragic ... (but) our job is not to tell people necessarily what they want to hear."

After waiting eight months for the decision, Hamilton's family reacted with disappointment and anger. At an emotional news conference on the steps of the federal courthouse, family attorneys said they had called for a federal investigation. They also urged that protests be peaceful "so as not to dishonor Dontre's name and the Hamilton family name."

But Hamilton's brother Nate spoke bitterly, saying the family had "cried too long" and "we don't have to be the voice of reason."

"We need to stop the violence in our communities so we can get rid of these pigs that kill us," he said to shouts and applause. "Because that's what they are. They feed, they feed off of us. And we can't let them do that no more."

His remarks came just two days after two New York City police officers were ambushed in their patrol car. Police said that attack was carried out by a man who posted online about putting "wings on pigs."

Police Chief Edward Flynn said he was disturbed by Hamilton's choice of words. "I would like to chalk it up to the emotion of the moment, but we don't need people implying or expressing a need for violence against police," he said.

Jon Safran, a Hamilton family attorney, later said Nate Hamilton doesn't condone "any type of violence" and the family was dealing with "great anxiety and frustration."

Hamilton's family has led mostly peaceful protests in the months since his death. They took to the streets again Monday afternoon, with Nate Hamilton leading a crowd as it marched through intersections chanting "Arrest the police!" and "Whose streets? Our streets!" The group eventually dispersed without incident.

Later Monday evening, more protesters gathered in a pouring rain at the park where Manney shot Dontre Hamilton. Nate Hamilton told the group to remain peaceful because media outlets are looking to broadcast violence and the protesters need to be smarter than that. "Don't nobody be violent," he said.

That crowd left peacefully as well.

About eight hours after Chisholm announced his decision, U.S. Attorney James Santelle said the Department of Justice, along with his office and the FBI, will conduct a review to determine if there was a violation of federal civil rights law. Manney's attorney didn't return a message seeking comment.

Manney shot Hamilton, 31, after responding to a call of a man sleeping in a downtown park. Manney said Hamilton resisted when he tried to frisk him. The two exchanged punches before Hamilton got hold of Manney's baton and hit him on the neck, the former officer has said. Manney then shot Hamilton 14 times.

Several witnesses told police they saw Hamilton holding Manney's baton aggressively before Manney shot him, according to Chisholm's report. Police said they have no video of the incident.

Chisholm consulted with police use-of-force experts, who concluded Manney's conduct was justified. Emanuel Kapelsohn of the Peregrine Corp. said all the shots were fired in 3 or 4 seconds and there was no evidence that Manney continued firing after Hamilton hit the ground.

Manney suffered minor injuries, including a bite to his right thumb, a neck strain and a neck contusion, the report said. He was treated for post-concussion syndromes, a mild traumatic brain injury and had physical therapy for bicep and rotator cuff injuries, the report said.

Flynn fired Manney in October. He said then that Manney correctly identified Hamilton as mentally ill but ignored department policy and treated him as a criminal by frisking him.

Hamilton's family said he suffered from schizophrenia and had recently stopped taking his medication.

The Milwaukee Police Association condemned Manney's firing as a political move, and members voted no confidence in Flynn soon after the firing, which Manney has appealed. On Monday, the union praised Chisholm's decision, saying the officer had "no other option" in the situation.

Hamilton's death preceded the killings of Michael Brown in Ferguson, Missouri, and Eric Garner in New York City, but the case hasn't attracted as much attention as those fatal encounters with police.