In the racially divided city of St. Louis the chief prosecutor has embraced a controversial tool to hold police accountable: blacklisting cops who she says are too untrustworthy to testify in court.

So far, Kim Gardner has dropped more than 100 cases that relied on statements from the 29 officers who got on the list for alleged lying, abuse or corruption. And she won’t accept new cases or search-warrant requests from them, either.

From Philadelphia to Houston to Seattle, district attorneys recently elected on platforms of criminal justice reform are building similar databases of their own. Often known as “do not call” lists, they are also called “exclusion lists” or “Brady lists” after a famous Supreme Court decision requiring prosecutors to disclose to defense lawyers information about unreliable police officers or other holes in their cases.

The goal is to help prosecutors avoid bringing cases built on evidence from officers who are likely to be challenged in court, these new DAs say. Having a centralized list at a district attorney’s office, they say, allows for the gathering of institutional knowledge, so that if one prosecutor on staff knows about a bad cop, all the prosecutors do.

But the strategy has infuriated police unions and some law-enforcement officials, who say they should get a say in who’s named on the lists—or else crime victims will pay the price.

“Kim Gardner is saying that when you dial 911, you’re playing 911 roulette: You may get an officer who’s on her list and who can’t give you justice,” said Jeff Roorda, spokesman and business manager for the St. Louis Police Officers’ Association.

Prosecutors who’ve adopted exclusion lists counter that victims rely on officers to be able to testify about the evidence they’ve collected and the witnesses they’ve interviewed without being challenged on integrity grounds.

Supporters of the lists say that officers are typically included based on documented instances of lying on the stand or in their police reports, not rumors. And under police-friendly state laws and employment contracts negotiated by powerful unions, cops have a variety of legal opportunities to clear their names.

“As elected prosecutors, we have the discretion to choose whether to entertain certain cases from certain individuals,” Gardner said in an interview. “Our obligation is to evaluate the credibility of any witness, regardless of whether they're police.”

“We have people whose liberty is at stake,” she said.

Exclusion lists are not entirely a new phenomenon. But in the past, they worked more like a grapevine, with anecdotes about cops shared among line prosecutors case-by-case. DA’s offices would discreetly inform police departments about officers with credibility issues, and those individuals would just as discreetly be reassigned to desk jobs.

In a city like Baltimore, which struggles to maintain police credibility in the black community, exclusion lists date back a decade. In 2008, the elected prosecutor, Patricia Jessamy, listed 15 current or former officers her office deemed not reliable enough to call as witnesses in court cases.

Police union leaders called the move unfair to officers, saying they are often subject to false complaints. Judging should be left to courts or administrative tribunals, they said. Prosecutors responded that those processes of determining guilt often took years.

Baltimore’s exclusion list forced the police to assign officers to desk duties, where they couldn’t make arrests or have other contact with the public. Due to the bad blood the policy created between cops and prosecutors, it was abolished in 2010 by Jessamy’s elected successor.

But the spectre of the list hangs over frigid police-prosecutor relations to this day. DAs still factor in the credibility of officers when mulling over cases, and the current prosecutor, Marilyn Mosby, is actively considering reconstituting a list, said her chief deputy, Michael Schatzow.

The Baltimore police union did not respond to requests for comment.

The city has dealt with a spate of high-profile allegations of police abuse and corruption, from the in-custody death of Freddie Gray in 2015 to a group of police officers who were convicted in the last year of robbing people.

Especially in the black community, the lists can help reassure frustrated citizens who believe that too many police officers have gotten away with misconduct, proponents say. While landing on a list may not get an officer fired, it is a roadblock to advancement.

“There has to be some level of balance where you’re not ostracizing police officers who are working hard at making their cities and communities safer, and also some way for a city to not go through what Baltimore has had to deal with,” said Baltimore City Councilman Brandon Scott.

There are challenges to implementing do-not-call databases successfully. Officers who have committed wrongdoing in other states or counties may be difficult to identify, for example.

And in many parts of the country, prosecutors are not given full access to police departments’ files. In a handful of states, they are banned from seeing certain disciplinary records altogether, effectively preventing many DAs from maintaining a comprehensive list of bad cops.

Prosecutors and some police officials note that DAs may not be aware of low-level efforts to mislead on the part of rogue officers; due to the sheer volume of cases that district attorneys’ offices handle, individual prosecutors say they can’t go back and reinvestigate everything the police are telling them. That means that the most everyday forms of dishonesty by officers (and crime lab employees) might never land them on an exclusion list.

Ronal Serpas, executive director of Law Enforcement Leaders to Reduce Crime & Incarceration, an advocacy group of more than 200 police chiefs and prosecutors, said a better solution than blacklists would be for district attorneys to urge police leaders to implement “one and done” policies. Such rules would require immediate firing for any work-related lie.

Serpas said he followed that policy when he ran both the Nashville and New Orleans police departments.

“You don’t need to have a ‘Brady no call’ list if the police department is terminating people,” he said.

Chris Magnus, the police chief in Tucson, Arizona, says that’s easier said than done.

“If I had my way, officers who lie wouldn’t just be put on a list, they’d be fired, and also not allowed to work in any other jurisdiction as a police officer ever again,” said Magnus, who says he hands over a list of problem officers to prosecutors. “But unfortunately, we have to allow them back into the workplace” due to union contracts.

“It frustrates the hell out of me,” he said, “that we have employees receiving full pay but who can’t really function as full police officers.”