A judge has ordered Yahoo to present a witness and provide documents explaining how the company handles supposedly deleted emails.

The move comes in the appeal case of a drug trafficker who was convicted, in part, because of emails Yahoo provided to law enforcement that conspirators believed had been deleted.

Defense lawyers in the case claim that six months of deleted emails were recovered—something which Yahoo's policies state is not possible. The defense therefore speculates that the emails may have instead been collected by real-time interception or an NSA surveillance program.

United States Magistrate Judge Maria-Elena James, from a San Francisco court, granted the defense's motion for discovery in an order filed on Wednesday.

The case revolves around Russell Knaggs, from Yorkshire, England, and a single Yahoo mail account. In 2009, Knaggs orchestrated a plan to import five tonnes of cocaine from South America. At the time, Knaggs was already serving a 16-year prison sentence for another drug crime.

As part of the operation, a collaborator in Colombia would log into the email account "slimjim25@ymail.com" and write a draft email. An accomplice based in Europe would then read the message, delete it from both the "draft" and "trash" folders, and write his own draft, in an effort not to leave behind any messages that could later be read by law enforcement.