CHICAGO (CBS) — Prosecutors agreed Thursday to overturn the convictions of 15 men whose cases were tied to a corrupt Chicago cop, marking the largest single day of exonerations in Cook County history.

Hundreds more exonerations could follow, as prosecutors continue to investigate allegations Police Sgt. Ronald Watts and his crew routinely framed suspects.

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Cook County prosecutors agreed to throw out convictions and drop charges in 18 cases brought against the 15 men, who were sent to prison on drug charges.

The newly exonerated former convicts showed up at the Leighton Criminal Courts Building on Thursday for what is believed to be the first mass exoneration in Cook County history. All 15 had filed a petition alleging Watts and his crew framed them between 2003 and 2008.

The Cook County State’s Attorney’s office said a review by its Conviction Integrity Unit “has shown a pattern of narcotics arrests that raises serious concerns about the validity of the resulting convictions for these 15 defendants.”

At least seven officers who worked on Watts’ tactical team are still on the Chicago Police Department, but those officers were stripped of their police powers Thursday night. They have been placed on desk duty while the department investigates possible wrongdoing.

Police Supt. Eddie Johnson says he’s taking a “hard look” at their conduct.

“Anytime you have cases where officers engage in misconduct, none of us like that kind of thing because it paints the entire department in a negative light,” he said Thursday at an appearance at the City Club of Chicago. “I’m not saying these officers are involved in that, I don’t know, but that’s why we’re taking a hard look at that.”

Joshua Tepfer, the lead attorney for the 15 men who were exonerated, said, “what’s never understood is that this stuff sticks with you.”

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“These convictions stick with you. You can’t get the time back you served,” he said. “It affects your ability to get jobs, housing.”

Leonard Gipson was only 22 when he was arrested on drug charges in 2003. He said Watts framed him after he refused to pay the sergeant’s so-called “protection tax.”

Gipson said Watts on three separate occasions planted heroin on him, and placed him under arrest. After he bonded out in the first case, Watts again planted drugs on him, and Gipson ended up pleading guilty and serving four months in boot camp.

In 2007, he ran into Watts’ crew again, and was arrested a third time after they planted more drugs on him. That case got him four years in prison.

“Watts always told me, ‘If you’re not going to pay me, I’m going to get you.’ And every time I ran into him, he put drugs on me. Every time,” Gipson said after his conviction was overturned.

Now exonerated, Gipson said he feels like a newborn baby with his whole life ahead of him. He hopes he can now get a job with his record cleared.

Four years ago, Watts and Officer Kallatt Mohammad were sentenced to nearly two years in prison, for stealing thousands of dollars from a drug suspect who turned out to be an FBI informant.

In their guilty pleas, Watts and Mohammed admitted to routinely extorting money from drug dealers.

Mark Rotert, head of the Conviction Integrity Unit, said State’s Attorney Kim Foxx is committed to reviewing other cases where Watts was involved.

“One of the priorities of this office in her administration is to restore trust between the criminal justice system and its actors and the citizens of Cook County,” he said.

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Tepfer said, in all, 26 convictions spearheaded by Watts and his crew have been overturned in recent years, involving a total of 20 defendants who now have been cleared. He said Watts was involved in more than 500 total convictions, leaving more than 400 still in question in his view.