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By Erin McClam and Elizabeth Chuck

Attorney General Loretta Lynch announced on Friday that the Justice Department would investigate whether the Baltimore police have engaged in a pattern of civil rights violations.

Mayor Stephanie Rawlings-Blake asked for the investigation after six police officers were charged in the death of Freddie Gray, who suffered a spinal cord injury in police custody. Rioting broke out in Baltimore after his funeral.

The Justice Department probe is known as a pattern-or-practice investigation. The attorney general rarely says no when a mayor or police chief asks for one, and the department can open such an investigation on its own.

Lynch said Friday that the federal probe would begin immediately: "If unconstitutional policies or practices are found, we will seek a court-enforceable agreement to address those issues. We will also continue to move forward to improve policing in Baltimore even as the pattern or practice investigation is underway.”

Here's a look at other notable pattern-or-practice investigations:

Los Angeles (2000)

Los Angeles erupted in chaos after four LAPD officers who were videotaped in the beating of Rodney King, a black man, in 1991 were acquitted of assault. The riots killed 53 people and put the spotlight on the city's racial tensions, and a pattern of police misconduct.

A frame from a video tape shot by George Holliday from his apartment in a suburb of Los Angeles shows what appears to be a group of police officers beating a man with nightsticks and kicking him as other officers watch on March 31, 1991.George Holliday Via KTLA Los Angeles / AP File

But it wasn't until 1996 that federal investigators started taking a hard look at the LAPD for excessive force violations. By May 2000, the Department of Justice was ready to sue the LAPD for misconduct. After lengthy negotiations, the city entered into a consent decree in November 2000, allowing for federal oversight as the police force implemented new policies. Longtime Police Chief Daryl Gates resigned, and then-Mayor Tom Bradley appointed an independent commission to examine the police department.

The department's response was widely seen as a blueprint for other departments around the country that had lost the trust of their communities.

Cincinnati (2002)

In April 2001, police shot a 19-year-old unarmed black man named Timothy Thomas to death, raising suspicions of racial profiling. The community reaction was strong: riots, a boycott of businesses, and increased crime. Distrust reached a fever pitch, particularly because the shooting came on the heels of a lawsuit claiming decades' worth of racial discrimination by officers.

More than a decade ago, the Rev. Damon Lynch III was one of the Cincinnati Police Department's biggest antagonists. Then he and other community leaders became a partners in a historic agreement to reform the police and rebuild trust among black residents.AL BEHRMAN / AP file

The Justice Department appointed a federal monitor for Cincinnati. Police and local civil-rights groups worked together to create an outline for reform, a response other struggling departments have turned to in times of crisis. The force then undertook a seven-year-long transformation under federal watch, which resulted in improved recruiting, better ties with the community, and an early warning system to identify officers who strayed from the new policies.

Cleveland (2014)

The Justice Department found a pattern of “unreasonable and in some cases unnecessary force” by Cleveland police. The city agreed to work toward a settlement that will provide for an outside monitor.

Federal investigators said the improper tactics included shootings, blows to the head and excessive force against the mentally ill, concluding that officers are not given adequate training and supervision.

Among the episodes that led to the investigation was the shooting of an unarmed man and woman in November 2012 after a high-speed chase. Thirteen officers had fired 137 shots.

Albuquerque (2014)

A 16-month Justice Department investigation into Albuquerque police was damning: Officers routinely violated residents' constitutional rights by beating them, Tasering them, even shooting them dead. The investigation reviewed 21 fatal police shootings between 2009 and 2012, including the killing of 19-year-old Andrew Lopez, shot multiple times after driving with no taillights in February 2009. Lopez's estate eventually received a $4.25 million settlement.

"Officers were not justified under federal law in using deadly force in the majority of those incidents," the federal report said. "Albuquerque police officers shot and killed civilians who did not pose an imminent threat of serious bodily harm or death to the officers or others."

Ferguson, Missouri (2015)

Attorney General Eric Holder opened an investigation after a white police officer shot and killed an unarmed black teenager, Michael Brown, last August. The results were explosive.

Police wearing riot gear walk toward a man with his hands raised in Ferguson, Mo, on Aug. 11, 2014.Jeff Roberson / AP File

The Justice Department, after reviewing 35,000 pages of records and conducting hundreds of interviews, concluded that Ferguson police routinely violated the Constitution, engaged in racial bias and focused on making money over public safety. Chief Thomas Jackson quit a week later, capping a string of resignations that included the city manager and two other police officers.