The Department of Justice announced Monday that it will investigate how police use force and handle allegations of misconduct in Chicago. So what happens next?

We can glean some insights into what Chicago could face from a Washington Post review of 20 years of civil rights investigations by the department. In November, The Post found that pushes for reform have led to a succession of police chiefs at the departments being targeted, and dragged on for years and cost hundreds of millions.

The Justice Department has launched 67 civil rights investigations since 1994, when Congress gave the federal agency the authority to investigate police departments in the wake of the Rodney King beating by Los Angeles police officers.

The Chicago Police Department, which has 12,000 sworn officers, is the country’s second-biggest local law enforcement agency and will be among the largest agencies the Justice Department has ever scrutinized for civil rights abuses.

The largest police agency that the department has imposed reforms on is the Puerto Rico state police department, which has 15,000 officers. In Puerto Rico, police are at the start of a projected 10-year, $200 million reform agreement after federal investigators found a broad range of civil rights abuses by officers.

Chicago shouldn’t expect the Justice Department’s investigation to be quick. Typical civil rights investigations conducted by the department have taken between 12 and 18 months, The Post found. The Justice Department has about 15 attorneys assigned to the investigations, each handling several cases at once.

While Mayor Rahm Emanuel forced Garry McCarthy to resign from his post as superintendent of police last week, before the investigation was announced, it’s not uncommon for police chiefs to depart once the Justice Department starts investigating.

In Detroit, the city cycled through eight police chiefs during the 11 years it took to impose federal reforms. In Los Angeles, the police force was led by seven chiefs during the nearly 12 years it took to overhaul the department.

The Justice Department has undertaken its deepest interventions at 16 police departments where it found patterns of excessive or deadly force, implementing reforms under the watch of independent monitors. Those 16 departments alone cycled through 52 police chiefs as the agencies tried to meet federal demands.

Among the 16 departments, only one police chief kept his job. That was in Pittsburgh, the first city the federal government targeted for intervention in 1997.

After the Justice Department completes its investigation of a police department, it issues a public letter detailing its findings. If warranted by the findings, the federal officials will then negotiate a settlement agreement, a process that can sometimes take years. Once the terms are negotiated, these police departments will typically be placed under the watch of an independent monitor who tracks reform progress.

Only one department — the sheriff’s office in Alamance County, N.C. — has successfully blocked the Justice Department’s attempts to impose reforms. Federal authorities are currently appealing that case.

Implementing the reforms sought by the Justice Department has often taken longer than projected. The agreements in Detroit and Los Angeles took twice as long to complete, and none of the departments reviewed by The Post completed reforms by the targeted dates.

The longer the delays, the greater the cost to taxpayers.

Los Angeles officials said they spent $300 million. Officials in New Orleans, where an agreement began in 2013, anticipate its four-year agreement will cost the city $45 million.

The reform agreements have led to new policies, better record-keeping and more training at the departments targeted. The Post found, however, that measured by the use of force, the outcomes have been mixed.

No one knows for sure the long-term impact of such agreements, because once the department reaches compliance with the reform agreement, the Justice Department’s involvement effectively ends.