The U.S. Justice Department will investigate the police force in Ferguson, Missouri, following the killing of an unarmed black male last month, the Associated Press reported.

Michael Brown, 18, was shot by a white police officer in a suburban neighborhood of Ferguson, sparking outrage and renewing criticism about racism in the country.

Since then, Gov. Jay Nixon was quoted as saying that the investigation into the incident at the local level required "vigorous prosecution" and "justice for the family" according to the St. Louis Post-Dispatch.

The phrases used by Nixon angered the more than 1,000 members of the Missouri Peace Officer's Association, who sent a letter written by former St. Louis County police chief Tim Fitch.

"These comments clearly do not reflect what society wants and law enforcement expects, which is a complete and thorough investigation, regardless who the victim or defendant may be. Your use of the phrase 'vigorous prosecution,' suggests guilt before due process, which results in no justice for the families, law enforcement and society," the letter said, according to the Post-Dispatch.

But Nixon replied that his comments were not an indication of pre-judgement.

He ended a three-week state of emergency, Wednesday, after putting the order into effect following the shooting.

The U.S. DOJ will be investigating the practices in the past few years of the Ferguson police department, including patterns of stops, arrests, use-of-force and training materials and exercises, AP reported.

This will be separate from a civil rights investigation into the shoot, also being conducted by the DOJ, into what occurred in the scuffle which broke out before the officer shot Brown.

Police have said Wilson was pushed into his squad car and physically assaulted, which contradicts witness accounts that say Brown's arms were in the air in an act of surrender.

This comes as no surprise to residents of Ferguson, which is about 70 percent black, and some have reported similarly disturbing accounts of conflict with police, AP reported

Previously, one man came forward saying he was charged with bleeding on an officer's uniform after being physically assaulted by them.

AP reported that some residents have said police disproportionately target black motorists, a fact that was confirmed based on a 2013 report by the Missouri attorney general's office.