Annie Dookhan sits in Suffolk Superior Court moments before her arraignment in Boston, Thursday, Dec. 20, 2012. Dookhan, the former chemist at the center of a U.S. drug testing scandal, pleaded not guilty to charges including perjury and tampering with evidence.

Thousands Of Potentially Wrongful Convictions; Years Of Delayed Action

Four years after a Massachusetts crime lab chemist confessed to tainting evidence, more than 20,000 defendants still don’t know if their drug convictions will stand.