Baltimore's top law enforcement and political leaders vowed a sweeping overhaul of the city police department after the Justice Department issued a searing rebuke of the agency's practices, which federal authorities say regularly discriminated against black residents in poorer communities.

Officials warned, however, that reforming a department entrenched in a culture of unconstitutional policing would be a slow process and could cost millions.

"Police reform won't happen overnight or by chance," Principal Deputy Assistant Attorney General Vanita Gupta said at a news conference unveiling the findings of the report Wednesday. "It's going to take time and it's going to require a focused and sustained effort."

Gupta said there were "long-standing systemic deficiencies" within the Baltimore Police Department and that "sustainable reform" was necessary to keep both officers and the community safe.

The sharp indictment of the agency came in a extensive report the federal government released this week after a 14-month "pattern or practice" investigation of the city's police force.

The probe found that a police force rooted in "zero tolerance" enforcement that started in 1999 but ended a decade ago has created a deep divide between police and many members of the community it serves. The city's policing strategy, lack of training and inattention to officer accountability has cultivated an agency that allows and encourages officers to stop, arrest or search black residents with little or no legal justification. The report also found that police department engaged in unnecessary force against juveniles, people with mental health issues and people who were restrained and presented to no threat.

"BPD deployed a policing strategy that, by its design, led to differential enforcement in African-American communities," the report stated. "But BPD failed to use adequate policy, training and accountability mechanisms to prevent discrimination, despite long-standing notice of concerns about how it polices African-American communities in the City."

In other words, according to the 163-page Justice Department report: "The relationship between the Baltimore Police Department and many of the communities it serves is broken."

Baltimore Mayor Stephanie Rawlings-Blake said while the findings of the report are "challenging to hear," the investigation creates a "crucial foundation" that will allow the city to change the department.

"It's so very important that we get this right," Rawlings-Blake said. "The report and its follow up will help to heal the relationship between the police and our communities."

Now that the investigation is complete, city officials will work with the justice department to implement a series of court-mandated reforms outlined in what is known as a "consent decree." The mayor said it could cost the city anywhere from $5 million to $10 million annually to make the suggested changes, which include improved training programs and new technology and equipment to modernize the police force.

The court-enforced order will be independently monitored and designed to sustain reform regardless of who is the police commissioner or mayor, justice officials said.

City police Commissioner Kevin Davis said that he has already fired some officers as a result of the Justice Department's investigation. Davis also said that he would not tolerate policing that is sexist, racist or discriminatory.

"Change is painful, growth is painful, but nothing is as painful as being stuck in a place that we don't belong," Davis said.

Baltimore has long struggled with strained relations between residents and police, but the need to ease those tensions became more urgent after the April 2015 death of Freddie Gray, 25. Gray suffered a fatal spine injury in police custody, triggering demonstrations and riots that flung the city into the national debate over race-based policing and fatal law encounters involving black men.

The same day the governor lifted the state of emergency in Baltimore following unrest over Gray's death, Mayor Stephanie Rawlings-Blake requested the civil right's probe from the Justice Department. Her request put Baltimore on the expanding list of cities - including Chicago, Ferguson, Missouri, and Cleveland - that have sought federal resources to enact law enforcement reform.

Baltimore State's Attorney Marilyn Mosby - whose office prosecuted six officers involved in Gray's arrest - said in a statement on Tuesday that the Justice Department's report "will likely confirm what many in our city already know or have experienced first hand."

"While the vast majority of Baltimore City Police officers are good officers, we also know that there are bad officers and that the Department has routinely failed to oversee, train, or hold bad actors accountable," Mosby said in a statement.

Mosby's office dropped charges against three of the six officers charged in the Gray case after a judge acquitted the first three officers who went to trial.

Since Gray's death, the city has already enacted a number of reforms, including installing new seat belts and cameras in the back of police vans and accelerating the city's body camera program.

Wednesday's report - which focusing on agencywide, institutional practices - is separate from a specific, ongoing probe into Gray's death.