A former Milwaukee police officer who shot and killed a black man in April will not face charges, the city's district attorney announced Monday, calling the gun use "justified self-defense." But the Justice Department will launch a civil rights review of the case, a U.S. attorney announced later in the day.

Christopher Manney, who is white, encountered Dontre Hamilton while Hamilton was sleeping in a downtown park on April 30. Manney, who was fired from the Milwaukee Police Department in October, was patting down Hamilton when a physical encounter ensued. About a dozen witnesses said punches were thrown and Hamilton eventually got hold of Manney's baton, according to the district attorney's report on the case.

Manney then fired 14 times, killing the 31-year-old Hamilton with shots to the chest.

"I believe there can be little serious doubt that P.O. Manney was justified in firing at Dontre Hamilton, who was attacking him with a deadly weapon," Milwaukee County District Attorney John Chisholm said in a statement Monday.

Hamilton's family has said he suffered from mental illness.

Milwaukee Police Chief Edward Flynn fired Manney in October for performing what he described as an inappropriate pat-down.

"My decision has nothing to do with whether it was OK for Officer Manney to pat down Mr. Hamilton," Chisholm noted while speaking to reporters on Monday. "My decision dealt with the use of force."

Later on Monday, the U.S. Attorney's Office for the Eastern District of Wisconsin said it would review the case to determine whether there is a basis for a federal civil rights prosecution. The office said it would conduct the review with the FBI and the Justice Department's Civil Rights Division.

The shooting occurred several months before other high-profile incidents in which white police officers killed unarmed black men. In Ferguson, Mo., a police officer killed 18-year-old Michael Brown. In New York, Eric Garner died over the summer after a white officer placed him in what appeared to be a chokehold.

Grand juries in both New York and Ferguson declined to indict the officers. The Milwaukee case was not heard by a grand jury.