The sweeping federal civil rights investigation by the US justice department will look at whether Ferguson police officers made discriminatory traffic stops, mistreated prisoners or used excessive force in the years before, Attorney General Eric Holder announced Thursday.
The small town of 22,000, nearly 70 percent of them black, with all but four of its 53 strong police force white, erupted into violent protests and looting with some Indian-American run stores among those targeted after the shooting of teenager Michael Brown.
Citing a "deep mistrust" between officers and the people who live there, Holder said the investigation would determine whether officers in Ferguson had "engaged in a pattern or practice of violations of the US Constitution or federal law."
He said the investigation will go beyond the shooting death of Brown, by a white officer, Darren Wilson, on Aug 9.
State statistics have also shown that blacks in St. Louis County, including Ferguson, are more likely to be pulled over and more likely to be arrested during traffic stops than whites, according to NBC News.
Holder said that his conversations with the people of Ferguson, when he visited in the aftermath of the Brown shooting, had shown the pattern of mistrust.
He said that Justice Department officials had also reviewed records, including racial statistics for police stops, before deciding to open the investigation.
The Justice Department is investigating the Brown shooting separately, as is a grand jury in the St. Louis area.
The Ferguson mayor and police chief both told NBC News on Thursday that they welcomed the federal investigation.
But the federal investigation will not cover the police response to the nightly protests in Ferguson that followed Brown's death, the New York Times reported.
Law enforcement officers, many clad in body armour and riding military-style trucks, descended on Ferguson from across the region, firing tear gas and pointing assault weapons at the protesters, the Times noted.
The influential newspaper cited Holder as saying officials from the Justice Department's Office of Community Oriented Policing Services were working with the police in St. Louis County to review the response to the protests as part of a "collaborative reform effort."
That effort will cover racial profiling, police response to mass demonstrations and other areas of concern.
While it is not a criminal review, the Times cited officials as saying they would refer any criminal behaviour they uncover to federal prosecutors.