Prosecutors in Massachusetts have dropped more than 21,000 low-level drug cases because of a drug lab scandal. A chemist at the lab admitted to tampering with evidence, forging test results and lying about it.

The chemist, Annie Dookhan, served three years in prison and was released last year, NBC News reports.

There is a second scandal at another drug lab in Massachusetts, involving a chemist who admitted to using drugs on the job, which has threatened thousands more convictions, the article notes. Thousands of convictions have been undermined by lab scandals in eight states in the last decade, according to the National Association of Criminal Defense Lawyers.