By BOB SALSBERG ,  Associated Press

BOSTON (AP) — Prosecutors in Massachusetts moved Tuesday to dismiss thousands of drug convictions tainted by a former state drug lab chemist who pleaded guilty to tampering with evidence and falsifying tests.
The state's highest court had ordered district attorneys in seven counties to produce lists by the end of the day on Tuesday indicating how many of the approximately 24,000 affected cases involving Annie Dookhan they would not, or could not, prosecute if new trials were ordered.
Dookhan pleaded guilty in 2013 to charges of obstruction of justice, perjury and tampering with evidence and was sentenced to three years in prison. She was paroled last year.
"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 American Civil Liberties Union of Massachusetts, in a statement.
The ACLU estimated that some 20,000 cases would be thrown out Tuesday, which the group said would make it the single largest dismissal of criminal convictions in U.S. history.
Most of the individuals involved had already served their sentences, prosecutors noted.
Bristol County District Attorney Thomas Quinn said his office was moving for dismissal of more than 1,500 cases, while seeking to maintain only 112 convictions.
"The actions of Annie Dookhan have imperiled the prosecution of thousands of drug cases throughout the commonwealth, including in some cases, defendants who presented a danger to the community," said Quinn.
The Essex County district attorney notified the Supreme Judicial Court it was dismissing all of the cases in district court with tainted evidence, and all but 55 of 150 Superior Court cases.
Prosecutors in Suffolk County, which includes Boston, and the state's largest county, Middlesex, had not yet filed their lists as of midafternoon. The other counties which must submit lists of cases to be dismissed are Plymouth County, Norfolk County and the Cape and Islands district.
The high court in January turned down a bid by the ACLU and public defenders to issue a blanket dismissal of all the potentially tainted cases, but also rejected prosecutors' arguments that a previous letter sent to more than 20,000 defendants last September had been sufficient.
More than 5,700 of those letters were returned as undeliverable, the court said, and the entire mailing resulted in motions for post-conviction relief in less than 1 percent of the total cases.
The justices instead proposed a multi-step process to resolve the cases, the first step requiring district attorneys to vacate cases they could not retry based on the evidence. For the remaining defendants, the prosecutors would be required to show the ability to try the cases without relying on evidence mishandled by Dookhan, and provide "adequate notice" to defendants of their right to explore a new trial or retract earlier guilty pleas.
Prosecutors said Dookhan admitted "dry labbing," or testing only a fraction of a batch of samples, then listing them all as positive for illegal drugs. Her motive, they said, was to burnish her productivity and reputation.
Even if most of the impacted defendants affected by Dookhan had already served their sentences, wiping their conviction will help them in other ways such as employment, housing or lawful immigration status, said Daniel Marx, an attorney who argued for the dismissals.
"Now, a majority of these wrongfully convicted individuals will have the opportunity to clear their records and move on with their lives," he said.