reporter: free because of the bizarre alleged actions of this woman, former massachusetts chemist annie ducanus what happened. >> reporter: the state of massachusetts is accusing her of tampering with drug evidence that could call into question at least 34,000 cases going back to 2003, 34,000. at the moment she faces only three charges. however, in boston alone, the d.a. estimates as many as 500 convicted felons could be set free. how big of a mess is this? >> at this point, susan, we don't know. >> reporter: at this lab, now closed by the state, ducan allegedly mishandled drugs seized by police for evidence at trial. she allegedly estimated the amount of drugs at times by simply looking at them and certified some drugs as cocaine that are now testing negative. she didn't just write down the wrong thing. prosecutors accuse her of doctoring evidence to change test results. >> she would take known cocaine from an area that she knew was cocaine and actually add them to the sample to make it cocaine. >> reporter: she is also charged with lying on the witness stand about the credentials on her res