An audit found that drugs, guns, and $400,000 cash went missing from the department's evidence room.
The scandal, which involves evidence dating back to 1999, means up to 400 cases may need to be dismissed due to lost or tainted evidence, according to Norfolk District Attorney Michael Morrissey.
Norfolk judges have already dismissed five of the drug cases this week, and even cases that have already been closed with plea deals could be thrown out, Morrissey told the Globe.
"We won't and don't use tampered evidence. It's that simple," he said. "We play by the rules, as painful as it is to let some of these people go."
Officials have not announced what prompted the investigation, but prosecutors have asked Attorney General Maura Healey to investigate whether crimes were committed.
The officer in charge of the evidence room, Susan Zopatti, killed herself in May after learning of the audit, the Globe reported. Shortly after her death, police recovered two of the missing firearms from Zopatti's home.
The audit, released at a press conference Wednesday, listed 4,709 pieces of narcotics evidence - mostly bags of cocaine - that disappeared. An additional 38 pieces of narcotics evidence were declared compromised by being opened, left unpackaged, or had some pieces missing, the Patriot Ledger reported.
Sullivan called the auditor's report "deeply troubling and unacceptable." He noted that while most of the firearms and cash have since been recovered, 12 guns and $140,000 remain missing.
The audit also declared more than 2,000 pieces of property evidence missing, and found video evidence left unlabeled and sexual assault test kits stored in a trailer.