Baltimore Mayor Stephanie Rawlings-Blake on Wednesday requested that the Justice Department investigate whether city police have engaged in a “pattern or practice” of excessive force, adding her city to a growing list of municipalities that have asked for federal help reforming local law enforcement agencies in recent years.

“We all know that Baltimore continues to have a fractured relationship between the police and the community,” said Rawlings-Blake (D). “I needed to look for any and all resources I could bring to my city to get this right for my community.”

Rawlings-Blake’s comments came the same day that Gov. Larry Hogan lifted a state of emergency that was imposed in the wake of violence that erupted in the city after the funeral of Freddie Gray, who died of injuries that occurred while in police custody.

Gray, 25, suffered a spinal injury April 12 after he was arrested and placed in the back of a police transport van, shackled but without a seat belt. He died a week later in a hospital. On Friday, six police officers were charged in his death.

U.S. Attorney General Loretta Lynch (left) meets with Baltimore Mayor Stephanie Rawlings-Blake. (Mark Dennis )

At a morning news conference, Rawlings-Blake noted a number of reforms that she said her administration has implemented at the department. While a similar request has been made by members of the City Council, and the city has invited a federal review in the past, the mayor said that more needed to be done.

“We have to get it right,” she said, adding that she plans to equip police officers with body cameras by the end of the year. “Failure is not an option.”

In lifting Baltimore’s state of emergency, Hogan (R) announced the departure of the last of 3,000 National Guard troops and 1,000 police officers from other jurisdictions who had been called in to quell several days of violence.

“We quickly brought calm back to the city,” said Hogan, who ticked off several of the places he visited in the city over the past week, including Sandtown-Winchester, the neighborhood where Gray was taken into custody.

The Justice Department has broad latitude in whether to conduct the investigation requested by Rawlings-Blake, but if federal officials accept the mayor’s request, the work will not be easy, as similar investigations in cities such as Los Angeles, Seattle, Cincinnati and the District have shown.

The probe would be launched by attorneys from the Justice Department’s Civil Rights Division. Based on similar investigations, they would probably review police policies, conduct community interviews and scrutinize data to determine whether the department engages in policing that is discriminatory or unconstitutional. After detailing its findings, the Justice Department and the city would negotiate an agreement outlining required reforms. Then an independent monitor — paid for by the city — would step in to ensure changes­ take effect in a timely manner.

It’s a process that could be long, contentious and costly.

“It was hard from beginning to end,” said former Cincinnati mayor Charles J. Luken, who invited the Justice Department to review the city’s department after police there killed a 19-year-old black man in 2001, igniting three days of riots. “You’re dealing with cultures that just don’t change very quickly.”

The Justice Department has had the power to investigate local police departments for more than two decades under the Violent Crime Control and Law Enforcement Act of 1994.

Federal officials said they have received Rawlings-Blake’s request . The Justice Department reportedly receives hundreds of complaints against police annually from community groups, politicians and police departments themselves.

“The Attorney General is actively considering that option in light of what she heard from law enforcement, city officials, and community, faith and youth leaders in Baltimore yesterday,” Justice Department officials said in a statement after the mayor’s news conference.

On Tuesday, Rawlings-Blake met with Attorney General Loretta Lynch, who also gathered with community leaders and Gray’s family during a visit to the city.

Since the Civil Rights Division took charge of policing the police, the Justice Department has intervened in about 30 different jurisdictions, according to a report published by the Police Executive Research Forum. In most investigations, the federal government has been asked to examine unlawful stops and searches, biased policing or improper use of force.

Ferguson, Mo., and Cleveland, where police officers fatally shot 18-year-old Michael Brown and 12-year-old Tamir Rice, respectively, are the most recent municipalities to undergo the sort of probe Baltimore’s mayor requested. Officials in Philadelphia and the District have also requested similar reviews in recent years.

Requests for comment from Baltimore Police Commissioner Anthony W. Batts were not returned.

Such “pattern or practice” investigations look at civil rights violations from an agency­wide perspective, focusing on institutional practices as opposed to the conduct of a few officers. They are separate from investigations of individual ­cases, such as probes into the death of Brown or Gray.

Samuel Walker, an expert on police accountability and such federal investigations, said the aim is to create systemic change. He compared the process to cleaning up a rotten barrel of apples instead of simply tossing out a few bad apples.

“It does relatively little good to simply fire a couple of bad officers,” said Walker, a professor in the School of Criminology and Criminal Justice at the University of Nebraska at Omaha.

The civil rights investigations can also lend credibility to reforms where communities already have a deep distrust of the police department.

Former D.C. police chief Charles H. Ramsey called in the Justice Department because “the community did not have confidence that we could fix the problems on our own,” he told other top law enforcement officials at a summit on the topic in 2013.

In the past, investigations with the Justice Department have lasted five years or more.

“People need to understand that . . . they’re not going to see instant reforms,” Walker said. “It involves changing an organization, which is extremely difficult, and Baltimore has some serious problems.”