The Justice Department launched a broad investigation Thursday into the police department in Ferguson, Missouri, following the shooting last month of an unarmed black 18-year-old by a white police officer.

The investigation, which goes beyond an existing federal probe into the Aug. 9 shooting, will look for patterns of discrimination within the predominantly white department and focus on how officers use force, search and arrest suspects, and treat inmates at the city jail. The police department said it welcomed the investigation.

In announcing the action, Attorney General Eric Holder, who visited the St. Louis suburb two weeks ago, said he and his department had heard numerous concerns from people there about police practices, a history of “deep mistrust” of law enforcement and a lack of diversity on the police force.

The inquiry is part of a broader Justice Department effort to investigate troubled police departments and, when pervasive problems are found, institute changes. The department says it has investigated more than 20 police departments in the past five years, more than twice the number of cases opened in the previous five years.

Besides the investigation into the Ferguson police force, the Justice Department says it will also work with the St. Louis County police department, which trains officers from Ferguson and other local departments, to review the use of force, the handling of mass demonstrations and other aspects of policing. It will also conduct a report on the county’s response to the two weeks of sometimes violent demonstrations that followed the shooting.

Police have said the shooting came after a scuffle that broke out after Wilson told Brown and a friend to move out of the street and onto a sidewalk. Police say Wilson was pushed into his squad car and physically assaulted. Some witnesses have reported seeing Brown’s arms in the air before the shooting in an act of surrender. An autopsy paid for by Brown’s family concluded that he was shot six times, twice in the head.

The FBI is conducting a civil rights investigation into the shooting and a local grand jury is also evaluating the case.

The investigation announced Thursday will focus on a police department that is predominantly white, even though Ferguson is about 70 percent black. Some in Ferguson have said police disproportionately target black motorists during traffic stops, something Holder said particularly concerned him.

A 2013 report by the Missouri attorney general’s office found that Ferguson police stopped and arrested black drivers nearly twice as often as white motorists but were also less likely to find contraband among the black drivers.

Sen. Claire McCaskill, D-Mo., called the investigation “a step in the right direction.” Civil rights advocates championed the move, too, but said more needs to be done.

“On a national level, we will remain steadfast in our commitment to eliminate police brutality everywhere and ensure the rights of those most impacted by these practices are protected in their entirety,” said Tef Poe, a St. Louis artist and organizer with the group HandsUpUnited.

“This is an important step,” Poe said. “However, we know much more needs to be done in order to bring the officer who killed Mike Brown to justice and address the epidemic of deadly police violence across the country.”

The Justice Department’s civil rights division routinely investigates individual police departments when there are allegations of systemic use-of-force violations, racial bias or other problems. The department says it is currently enforcing 14 agreements to overhaul police department practices.

The investigations typically encourage significant changes to policies and practices and often end with settlements known as consent decrees in which the department agrees to make specified reforms.

“They will comb records of citizens’ complaints, they will look at the filing of lawsuits, they will look at all of the record-keeping in the police department,” said David Harris, a police practices expert at the University of Pittsburgh law school.

The Justice Department reached a court-supervised agreement in 2012 with the New Orleans Police Department that would require the agency to overhaul its policies and procedures for use of force, training, interrogations, searches and arrests, recruitment and supervision.

In April, it issued a harshly critical report of the police department in Albuquerque, New Mexico, that faulted the agency for a pattern of excessive force and called for an overhaul of its internal affairs unit. The city and the Justice Department have been locked in negotiations over ordered changes.

(AP)