Attorney General Loretta Lynch spoke 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)

Attorney General Loretta E. Lynch has decided to launch a federal investigation into whether the Baltimore Police Department has engaged in a “pattern or practice” of excessive force.

Lynch’s announcement about the Justice Department’s probe — the latest in a string of municipalities that are being investigated by the federal government for civil rights violations — could come as early as Friday, according to two law enforcement officials.

A Justice Department spokeswoman declined to comment Thursday night.

Lynch hinted Thursday that a decision could come soon, when she testified during her first hearing on Capitol Hill as attorney general. She said that she would decide “in the coming days” whether to have the department’s civil rights division open an investigation into the Baltimore police force.

Baltimore Mayor Stephanie Rawlings-Blake on Wednesday called on the Justice Department to open a federal investigation. Rawlings-Blake made the request after Baltimore State’s Attorney Marilyn J. Mosby filed criminal charges against six Baltimore officers who were involved in the arrest of Freddie Gray, who died of injuries sustained while he was in police custody.

Baltimore Police Commissioner Anthony W. Batts welcomed any federal review. “I think it’s a good thing,” Batts told WBAL-TV on Thursday after a conference of police chiefs. “We could use the extra weight. Lawsuits are down. Citizen complaints are down. Officer-involved shootings are down. But the community doesn’t feel it.”

Mosby said Gray was arrested illegally, was treated callously by the officers and suffered a severe spinal injury April 12 in the back of a police van while his pleas for medical help were ignored.

Gray’s death April 19 ignited protests and rioting in Baltimore and is the latest case to spark anger about law enforcement tactics in low-income neighborhoods across the country.

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

In her first official trip as attorney general, Lynch visited Baltimore on Tuesday 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. Vanita Gupta, acting assistant attorney general for the civil rights division, has also been meeting with Baltimore officials since the outbreak of violence after Gray’s funeral April 27.

“The situation in Baltimore involves a core responsibility of the Department of Justice — not only to combat illegal conduct when it occurs but to help prevent the circumstances that give rise to it in the first place,” Lynch told the Senate Appropriations subcommittee on commerce, justice, science and related agencies. “When there are allegations of wrongdoing made against individual officers and police departments, the Department of Justice has a responsibility to examine the evidence and, if necessary, implement changes.”

Since 1994, the Justice Department has been able to investigate local police departments under the Violent Crime Control and Law Enforcement Act. The departments in Ferguson, Mo., and Cleveland are undergoing federal civil rights investigations. A white police officer in Ferguson fatally shot unarmed teen Michael Brown in August, and 12-year-old Tamir Rice was shot and killed by a Cleveland police officer in November.

Baltimore resident Kevin Moore captured video of Freddie Gray being loaded into a police paddy wagon on April 12. In this video, Moore reflects on what he witnessed and talks about the charges brought against six officers in the death of Gray. (DeNeen Brown and Ashleigh Joplin/The Washington Post)

Over the past two decades, some city officials have asked the Justice Department to investigate and help reform their departments, as Rawlings-Blake did this week.

Former D.C. police chief Charles H. Ramsey called in the Justice Department after a 1998 Washington Post series found that D.C. police had shot and killed more people per resident in the 1990s than any other large American city police force.

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 civil rights investigation into the death of Brown; and the ongoing probe into the death of Eric Garner, an African American man who died after being placed in a chokehold by a white officer on Staten Island.

Ramsey, now Philadelphia’s police commissioner, said at a law enforcement gathering two years ago that he asked the federal government for help because the D.C. community “did not have confidence that we could fix the problems on our own.”

Sen. Barbara A. Mikulski (D-Md.) made the same point during the congressional hearing with Lynch on Thursday morning when she said she supported the mayor’s request to open an investigation. She sent a formal letter to the attorney general Thursday afternoon.

“Last year, 120,000 police stops occurred in Baltimore,” Mikulski said. “We’re a population of 610,000. That’s a lot. I don’t know what the appropriateness of that is, but I think we need to look at it.

“But I want to say this,” Mikulski added. “In many cities throughout the country, and including my own town of Baltimore, and in communities primarily that have significant populations of color, there has been now a tattered, worn and even broken trust between the community and the police department. We’ve got to restore that trust.”