FBI agent Matthew Lowry checked out Item 1B4 from the evidence room at the bureau’s Washington field office on an August morning in 2013. He wrote “to lab” on a log sheet to explain why he was taking drugs that had been seized in an undercover operation dubbed Midnight Hustle.

But it was nearly a year later when he delivered the drug package to the lab. For 10 months, court records show, the heroin had gone unaccounted for and unmissed. When the package made it back to the FBI office in September, it weighed 1.1 grams more than when it had been seized.

Someone had tampered with the contents, prosecutors said later.

The package is one piece of a tale that has unfolded in recent months — the case of an FBI agent who, by his own admission, repeatedly stole heroin from evidence bags for his personal use. In the process, he sabotaged drug cases that he and his colleagues had labored on for months. Prosecutors have dismissed charges against 28 defendants in three cases, many of whom had already been convicted, and they say it could affect 150 other defendants.