A St. Louis County grand jury on Monday decided not to indict police officer Darren Wilson for the shooting of Michael Brown, but the U.S. Justice Department’s civil rights division is still reviewing the incident and investigating allegations of unconstitutional policing practices in Ferguson, Mo.

Attorney General Eric Holder said in a statement Monday that federal civil rights law “imposes a high legal bar in these types of cases,” but he added that the Justice Department has “resisted forming premature conclusions.” Such investigations commonly take months to complete.

Holder also called for a “national conversation” to address mistrust between police and the communities they serve. He said the Justice Department will work with law-enforcement agencies, civil rights groups and faith and community leaders to “foster effective relationships.”

The civil rights investigation will examine whether the Ferguson police department has routinely engaged in racial profiling or a pattern of excessive force, according to a Washington Post article in September.

The review will also look for possible racial profiling and assess stops, searches, frisking and how the St. Louis County Police Department handled mass demonstrations that turned violent after the shooting.

The St. Louis County Police Department, which trains Ferguson officers, has volunteered to take part in a “collaborative reform effort” with the Justice Department, which could eventually lead to local law-enforcement officials agreeing to address any questionable practices that the review identifies.

On Monday, Holder urged police to respect protesters’ rights and “deescalate tensions by avoiding excessive displays — and uses — of force.” He also called for calm among individuals who disagree with the grand jury’s decision.

“Those who decide to to participate in demonstrations should remember the wishes of Michael Brown’s parents, who have asked that remembrances of their son be conducted peacefully,” Holder said. “It does not honor his memory to engage in violence or looting.”

Despite that statement and similar remarks from President Obama after the grand jury decision was announced, protests in Ferguson turned violent Monday night.