In the spring of 2010, two top officials with the U.S. Justice Department sat down at the University of Chicago Law School, where a group of professors and lawyers made a case for why the federal government should investigate the Chicago Police Department for widespread civil rights violations.

The closed-door conversation with then-Assistant Attorney General Thomas Perez and his deputy, Roy Austin, went on for more than an hour, said two people at the meeting, University of Chicago law professor Craig Futterman and Flint Taylor, a veteran civil rights attorney. It came after the group sent a detailed letter arguing that the police department had grown into an overly aggressive agency with deep-rooted patterns of false arrests and a history of targeting black residents. The federal government declined to take action.

“We presented evidence of a powerful case of the dire need for federal intervention,” said Futterman, director of the law school’s Civil Rights and Police Accountability Project. Department of Justice spokeswoman Dena Iverson declined to comment, saying the agency does not make its decision-making process public.

That wasn’t the first time such a request had been made.

In 1999, Attorney General Janet Reno did not move forward with a probe after an appeal from Taylor and Rep. Robert Rush, D-Ill. Reno did not respond to a request for comment. More recently, Cook County Commissioner Richard Boykin, a Democrat and vocal critic of Mayor Rahm Emanuel, sent letters earlier this year to Attorney General Eric Holder and his successor, Loretta Lynch, asking for federal intervention. The letters went unanswered.

After spending nearly a generation fielding complaints about the Chicago police force, the Department of Justice announced Dec. 7 that it had started a formal investigation into the department. It is operating under a 1994 law that allows the attorney general to sue local law enforcement agencies in federal court over allegations of unconstitutional behavior. The Chicago probe was the 23rd so-called pattern-and-practice investigation launched under President Barack Obama, according to The Marshall Project’s tally.

For many, the only question was, what took them so long?

The decision topped off a tumultuous month that began when a Cook County judge ordered the release of an October 2014 video that showed a white police officer killing a black teenager, Laquan McDonald. Emanuel fired his police superintendent, Garry McCarthy, and has been battered by calls to step down himself.

Futterman speculated that the Justice Department's inaction could be tied to several issues, including staffing shortages and politics. Taylor was more direct.

“Chicago was a place that the Justice Department didn’t want to investigate because it was such a Democratic stronghold,” said Taylor, who co-founded the People's Law Office and has represented hundreds of victims of police abuse.

Chicago has struggled for decades with conflict between its police and citizens, famously captured in the televised clashes at the 1968 Democratic Convention and in violent confrontations between the Chicago police force and members of the Black Panthers.

But the problems have gone far beyond those infamous events. Since 2004, Chicago has spent more than a half billion dollars on police-related lawsuits. Police commander Jon Burge, who is white, ended a four-year federal prison sentence in February after he was convicted of running a decades-long torture ring, subjecting dozens of black men to electric shocks and burnings during interrogations. In July, Chicago was cited as the nation’s largest city with the highest count, per capita, of police shootings.

“Why did Loretta Lynch decide, with the drop of a video, to open an investigation, when these pattern and practices of the police department have existed since I was a young lawyer?” Taylor said.

Chicago hasn’t had a Republican mayor since the Great Depression. The Daley family, led by Richard J. Daley, held the mayor’s seat for nearly a half-century. Daley died in office in 1976 after serving 21 years. His son, also named Richard, then became Chicago’s longest-serving mayor, running the city for 22 years, until 2011. The youngest child of the Daley clan, William, served as former President Bill Clinton’s Commerce Secretary. He was also President Obama’s chief of staff, a position that was also once held by Emanuel. Obama’s political career is also rooted in Illinois, where he was a state senator before being elected to the U.S. Senate. Emanuel’s office did not respond to requests for comment.

The number of pattern-and-practice investigations declined under former President George W. Bush, who once told the nation’s largest police union: “I do not believe the Justice Department should routinely seek to conduct oversight investigations.”

Samuel Bagenstos, the civil rights division’s number two from 2009 to 2011, said: “I absolutely never saw any indication of any political influence in any investigation that I worked on, or was involved in, at DOJ in the Obama administration.”

Robert Driscoll, a former deputy attorney general in the Justice Department’s civil rights division under President George W. Bush, suggested that the department sometimes takes action only after public outcry following a highly publicized police incident.

“Unfortunately, many cases appear to be opened, not in response to a previously known ‘pattern’ of misconduct, but as a parallel to a controversial incident,” Driscoll wrote in a Dec. 9 letter to Sen. Ted Cruz, R-Texas, after testifying at a Senate subcommittee hearing chaired by the senator. In Chicago, “the decision to open an investigation now… leads to the perception that either the McDonald case alone constituted sufficient ground to investigate a 'pattern and practice,' or that a pattern and practice of violations had existed for sometime and was not investigated previously given the political clout of Mayor Emanuel, a political ally of the Obama Administration,” Mr. Driscoll wrote.

Former Justice Department officials who also worked under Obama said the agency has limited resources that prevent it from investigating all of the nation’s troubled police forces. Increased media attention after a specific flare-up, however, can pique Washington’s interest.

Over the years, the Justice Department has undertaken scores of preliminary investigations into police departments. If supervisors in the agency’s civil rights division think the allegations warrant a formal investigation, they assign division attorneys to write up a memo, which is then presented to the assistant attorney general.

The Department of Justice will not reveal how many preliminary investigations it has on file.

Boykin, the Cook County commissioner, said Justice Department officials could improve the process simply by picking up the phone. “I am outraged that the Department of Justice didn’t even respond to my inquiries,” Boykin said.

An earlier version of this story mischaracterized Professor Futterman's views on the timing of the Justice Department's decision to investigate the Chicago Police Department.