Attorney General Loretta E. Lynch speaks about the “frayed trust” between police and the community in Baltimore at a news conference, where she announced a federal investigation into the city’s police practices. (Department of Justice)

The Justice Department has opened a formal investigation into the Baltimore Police Department to determine whether officers have committed systemic constitutional violations, Attorney General Loretta E. Lynch announced Friday, putting the city’s force under rigorous federal scrutiny.

The broad investigation, known as a “pattern or practice” review, will focus on the police department’s stops, searches and arrests, its use of force, and the question of whether there has been a practice of discriminatory policing. The review was requested by Baltimore’s mayor and will be a “collaborative reform process,” Lynch said, adding that she was optimistic that federal and city officials could work together to create a “stronger, better Baltimore.”

The investigation is separate from the Justice Department’s criminal civil rights probe into the case of Freddie Gray, the 25-year-old man whose death while in police custody ignited protests and rioting in Baltimore. Gray suffered a severe spinal injury in the back of a police van while his pleas for medical help were ignored, according to Baltimore prosecutors.

“We’ve seen events change in Baltimore and become more intense over a short time,” Lynch said during a news conference at the Justice Department, her first since becoming attorney general. “It was clear to a number of people looking at the situation that the community’s frayed trust — to use an understatement — was even worse and has in effect been severed in relationship to the police department.

“We felt the best thing to do was to conduct an investigation to see if these issues arose to the level of federal civil rights violations,” she said.

Since 1994, the Justice Department has been able to investigate local police departments under the Violent Crime Control and Law Enforcement Act. The department has opened more than 20 investigations of police departments under the Obama administration, including in Ferguson, Mo., where a white police officer fatally shot unarmed teen Michael Brown in August.

In addition to gathering information directly from community members, pattern or practice investigations involve interviewing police officers and local officials and gathering information from public defenders and prosecutors. The investigations also involve observing officer activities through ride-alongs and reviewing documents and specific incidents relevant to the probe.

Federal “pattern or practice” probes focus on the entire police department, rather than investigating the conduct of certain officers.

In that way, they are different from civil rights investigations of certain individuals, such as the two-year-long probe into the death of Trayvon Martin, the 17-year-old African American from Florida who was unarmed when he was fatally shot by a former neighborhood watchman.

The Baltimore riots broke out just as Lynch was taking over the Justice Department. On Friday, asked about her reaction to the violence, Lynch said she was deeply saddened.

“It was profound sadness for the loss of life, the erosion of trust, for the sadness and despair the community was feeling, for the frustration that I know the police officers were feeling as they tried to encourage peaceful protests and had to deal with violence,” she said.

Baltimore’s police union issued a statement welcoming the review. But in a sign of the tension between police and city leaders, the union also encouraged the Justice Department to expand its review to focus on Mayor Stephanie Rawlings-Blake (D).

Attorney General Loretta E. Lynch testifies on Capitol Hill on Thursday. (Susan Walsh/AP)

Rawlings-Blake issued her own statement that she was pleased by the Justice Department’s decision, saying: “Our city is making progress in repairing the fractured relationship between police and community, but bolder reforms are needed and we will not shy away from taking on these challenges.”

Lynch visited Baltimore earlier this week and met with the mayor, law enforcement officials and community leaders. She also met with Gray’s family and spoke with an officer who was injured in the violence.

In her remarks Friday, the attorney general said Baltimore has struggled with many of the same issues facing other cities.

“We have seen the tragic loss of a young man’s life. We have seen a peaceful protest movement coalesce to express the concern of a beleaguered community,” she said. “We have seen brave officers upholding the right to peaceful protest, while also sustaining serious injury during the city’s unfortunate foray into violence. And we have watched it all through the prism of one of the most challenging issues of our time: police-community relations.”

It’s unclear how long the investigation into the Baltimore Police Department will take. At its conclusion, federal officials will issue a public report. If they determine that the department has exhibited a pattern or practice of violating constitutional rights, they will seek a court-enforceable agreement aimed at restructuring the force.