The fatal shooting of black man Dontre Hamilton by a police officer in Milwaukee shall be reviewed by the US Justice Department.
It comes after the local prosecutor for the city decided not to press charges against officer Christopher Manney.
Street protests followed Hamilton's death in April, which is the third high-profile shooting of a black man by police to spark publice anger.
The US Justice Department probe shall investigate the police shooting for possible civil rights violations, after Manney shot Hamilton, 31, a total of 14 times.
A spokesman said the federal probe shall be: "A review of this case to determine if, under federal civil rights law, there is a basis, both legal and factual, upon which a federal civil rights prosecution may be premised."
Hamilton - who had schizophrenia - died after staff at a Starbucks coffee shop called police to report him for sleeping on a nearby park bench. Officer Manney claimed he was punched by Hamilton during a confrontation, after trying to frisk him. He then shot Hamilton.
Explaining the decision to not bring charges, prosecutor John Chisolm said the cop had used: "Justified self-defense and that defense cannot be reasonably overcome to establish a basis to charge Officer Manney with a crime."
Manney was dismissed from the police service over the incident and has appealed against the decision.