A call is being made for the appointment of an independent investigator to oversee the probe of the testing scandal at a state drug lab.

The Massachusetts Bar Association and the American Civil Liberties Union were among the groups to sign a letter sent on Wednesday to Attorney General Martha Coakley, asking her to appoint the outside investigator to fully examine what happened at the lab.

A former chemist at the lab, Annie Dookhan, has been accused of mishandling drug samples she tested, potentially jeopardizing thousands of criminal cases in Massachusetts.

Martin Healy, chief legal counsel for the bar association, said the crisis calls for a wider investigation to restore public confidence.

Said Matthew Segal, the ACLU’s legal director: “The investigation of the drug lab is going to require investigating not only state employees at the lab itself but also potentially prosecutorial officials and law enforcement officials.

“It’s important that this independent investigator be somebody that has the authority to track down this scandal, wherever it leads, and that authority would include subpoena power,” Segal added.

A spokesman for Coakley said the letter was under review, but added the office was surprised by the letter because it had been working with defense attorneys to conduct a “fair, complete and transparent” probe.

With reporting by The Associated Press and the WBUR Newsroom