Out of Chicago's roughly 12,000 police officers, just 124 were responsible for nearly one third of the force's misconduct lawsuits settled since 2009, costing the city $34 million and raising fresh doubts about the beleaguered department's ability to keep tabs on officer conduct.

Chicago's police department has been under constant scrutiny since last fall, as outrage over the videotaped shooting of 17 year-old Laquan McDonald brought attention to an alleged pattern of excessive force and cover-ups, particularly towards black men. Officer Jason Van Dyke, who faces a first-degree murder charge in Mr. McDonald's killing, has pleaded not guilty.

Some Chicagoans have called for Mayor Rahm Emanuel to resign, and the US Department of Justice opened a civil rights investigation in December. In early January, the FBI announced that it was investigating another 2013 police shooting death in Chicago.

But the majority of the 1,100 lawsuits settled since 2009 deal with non-fatal incidents, and 85 percent were resolved for less than $100,000, which helped them stay out of the limelight, according to a new investigation from the Chicago Tribune. The lower amounts included complaints about officers unnecessarily injuring suspects and using racial slurs. The department does not need City Council approval for settlements less than $100,000.

Only 5 percent of settlements cost more than $1 million. But just one percent of officers, 124 "repeat offenders" named in three or four settlements each, cost the city $34 million.

"This has been an issue for decades and there is no question the department needs to do a better job identifying officers with problematic behavior to hold them accountable and restore trust in the police," Chicago Police Department spokesman Anthony Guglielmi told the Tribune.

Since video of McDonald's death was released in November, Mayor Emanuel has promised change, firing police superintendent Garry McCarthy, creating a six-person Task Force on Police Accountability, and improving officers' mental health training.

"I've asked five respected Chicagoans who are leaders in the criminal justice system to do a top-to-bottom review of the system," Emanuel said when he announced the creation of the task force on Dec. 1.

Facing similar crises in 2007, the department created an Independent Police Review Authority (IPRA) meant to keep officers accountable. Many critics, however, say IPRA has wound up shielding officers. The Tribune's investigation suggests that officers with multiple lawsuits against them were rarely disciplined, and frequently worked together. In its own investigations, IPRA dismissed many of the charges for witness' inconsistencies or insufficient evidence.

The police department, IPRA, and Chicago's Department of Law are studying "new ways in which all available information, from settlements to complaints to actual findings, can be better used to hold officers with problematic histories accountable," mayoral spokesman Adam Collins told the Tribune.

Some legal experts say that individual officer misconduct is a symptom of deeper system problems.

In December, criminologist Samuel Walker told The Christian Science Monitor that IPRA should look to New York City as a model of police reform.

"In Chicago, you’re looking at whether the officer did something wrong or not,” Mr. Walker, a professor emeritus at the University of Nebraska, told Nissa Rhee. "In New York, the auditor is really looking at the organization. They are looking at policies and practices that affect all officers. And you’re looking towards fixing the underlying problem."