Chicago (CNN) The drug convictions of 15 men in Chicago have been thrown out after they claimed they were framed years earlier by a police sergeant, who went to prison for a related crime, and his team of officers.

The dismissal of charges against the men, who together had 18 convictions, is believed to the first-ever mass exoneration in Cook County , the nation's second most populous county.

"In these cases, we concluded, unfortunately, the police were not being truthful, and we couldn't have confidence in the integrity of their reports and their testimony, and so, in good conscience, we could not see these convictions stand," Mark Rotert of the Cook County Conviction Integrity Unit said in a news conference.

Seven other officers have been placed on desk duty pending an internal review of more incidents, the Chicago Police Department told CNN. Those officers all worked under Watts, CNN affiliate WBBM reported

Rotert, of the state's attorney's office, said his team is now reviewing additional cases connected to drug convictions spanning several years.

'They would frame them'

The men whose charges were tossed alleged that Watts and his team of officers planted drugs on them during arrests between 2003 and 2008, then falsified police reports, leading to their convictions, according to the Exoneration Project , a free legal clinic at the University of Chicago Law School that presented the cases to county prosecutors.

A Cook County judge on Thursday approved prosecutors' decision to drop the charges. All the men had served their sentences for the crimes in question, according to the Exoneration Project: 14 are free, and one remains incarcerated on unrelated charges.

One of the men, Leonard Gipson, said he was framed by Watts on drug charges and went to jail twice because of the officer.

"If you're not gonna pay Watts, you were going to jail," Gipson recalled. "I went to jail and did 2 years and 24 months for Watts. I came home, and he put another case on me."

Gipson's attorney, Joshua Tepfer of the Exoneration Project, said officers took money from and charged dozens of people for crimes they did not commit.

"They were skimming off people, and anyone who would get in their way, they would frame them," he told CNN. "And anyone who tried to report them, they would frame them."

CNN could not immediately reach Watts or Mohammed, who both have served their sentences.

'Ongoing, systemic, day-to-day corruption'

It wasn't until the summer of 2015, though, that Tepfer, the law clinic attorney, learned of the case of Ben Baker and started investigating.

"What I learned is what Ben Baker said happened to him -- that he was framed by this officer who was named Sgt. Ronald Watts, and what I learned right away was that Ronald Watts had been convicted federally of exactly what Ben said had happened to him: being shaken down and framing someone for putting drugs on someone."

"I started to look into it, and what we slowly realized this was ongoing, systemic, day-to-day corruption by not just Sgt. Watts and not just by his federally convicted co-defendant, Officer Kallatt Mohammed, but by a whole crew of officers that went on with impunity for over a decade on the south side Chicago housing projects."

Baker was freed last year after nearly 10 years in prison when the state's attorney dropped drug charges that Baker claimed were concocted by Watts and his crew, CNN affiliate WLS reported

"It was torture," Baker told CNN of his jail time. "You're thinking in your head: How did this happen? How did I go from laying next to my wife and going out with my kids every day to being in this cell every day?"