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BOSTON (Reuters) - Close to 20,000 Massachusetts criminal drug cases are set to be dismissed because of a scandal involving a former state chemist who admitted faking tests, civil liberties activists and prosecutors said on Tuesday.

It will mean the largest number of drug cases tossed out in U.S. history due to one person, according to the American Civil Liberties Union. Rogue chemist Annie Dookhan pleaded guilty in 2013 to tampering with evidence during her nine years working at a state crime lab in Boston. The scandal shook the foundation of the state’s criminal justice system.

The Massachusetts Supreme Judicial Court in January ordered prosecutors across the state to dismiss the vast majority of convictions tied to that lab, where Dookhan identified evidence as illegal narcotics even without testing it, in an effort to make herself seem more efficient.

“Today is a major victory for justice and fairness, and for thousands of people in the commonwealth who were unfairly convicted of drug offenses,” said Matthew Segal, legal director of the ACLU of Massachusetts, which represented many of the defendants during the appeal process.

“The victims of this crisis waited far too long for justice. It shouldn’t have taken years of litigation by the ACLU, public defenders, and pro bono lawyers to address this stain on the Commonwealth’s justice system,” Segal said.

Prosecutors in Massachusetts’ Suffolk County, which includes Boston, said on Tuesday they had opted to stand by the prosecution of just 1.5 percent of the cases they had brought involving Dookhan.

“The average defendant has more than 60 entries on his record,” County District Attorney Daniel Conley said, referring to the cases his office would still pursue. “They are neither low-level nor non-violent, and they stand at the intersection of drugs and violence.”

Those cases also rely on evidence not linked to the now-shuttered lab where Dookhan worked, Conley said.