MILWAUKEE -- A white Milwaukee police officer who was fired after he fatally shot a mentally ill black man in April won't face criminal charges, the county's top prosecutor said Monday.

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

"This was a tragic incident for the Hamilton family and for the community," Chisholm said in a statement. "But, based on all the evidence and analysis presented in this report, I come to the conclusion that Officer Manney's use of force in this incident was justified self-defence and that defence cannot be reasonably overcome to establish a basis to charge Officer Manney with a crime."

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. Hamilton's family has led mainly peaceful protests.

The Hamilton family released a statement through their attorney saying they were "extremely disappointed" with the decision and that the case "cries out for justice, criminal charges against Christopher Manney, and accountability to Dontre Hamilton's family."

The family said it has asked the U.S. attorney in Milwaukee to seek a federal investigation.

Manney's attorney did not immediately return a message seeking comment.

The executive director of the American Civil Liberties Union of Wisconsin, Chris Ahmuty, issued a statement saying the decision not to charge Manney left "a cloud of uncertainty over the circumstances of and the responsibility for Mr. Hamilton's death."

Manney shot 31-year-old Hamilton on April 30 after responding to a call for a welfare check on a man sleeping in a downtown park. Manney said Hamilton resisted when he tried to frisk him. The two exchanged punches before Hamilton got a hold of Manney's baton and hit him on the neck with it, the former officer has said. Manney then opened fire, hitting Hamilton 14 times.

Several witnesses told police they saw Hamilton holding Manney's baton "in an aggressive posture" before Manney shot him, according to Chisholm's news release.

Chisholm consulted with two experts on the use of force by police officers, and both concluded Manney's conduct was justified. Emanuel Kapelsohn of the Peregrine Corporation concluded that all the shots were discharged in 3 or 4 seconds and there was no evidence that Manney continued firing after Hamilton hit the ground.

Police have no video of the event.

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

Police Chief Edward Flynn fired Manney in October. He said at the time that Manney correctly identified Hamilton as mentally ill, but ignored department policy and treated him as a criminal by frisking him.

The Milwaukee Police Association condemned Manney's firing as a political move, and members voted no confidence in Flynn soon after the firing. Manney has appealed his dismissal.

Wisconsin Gov. Scott Walker has said he will call up the National Guard if there is any violent reaction to the prosecutor's decision. Police arrested 74 protesters Friday who blocked rush hour traffic on Interstate 43.