Defense lawyers claim that six months of deleted emails were recovered—which Yahoo's policies state is not possible.
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 "firstname.lastname@example.org" 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.
The defense alleges there should have been nothing for law enforcement to find
Sukhdev Thumber, a solicitor representing Knaggs in the UK proceedings, previously told Motherboard that the pair would sometimes simply remove the text in the draft with the backspace key, rather than deleting the email. Knaggs didn't actually use the account himself.
After receiving requests from UK police and the FBI in September 2009 and April 2010, Yahoo created several "snapshots" of the email account, preserving its contents at the time—and revealing the messages. But the defense alleges there should have been nothing for law enforcement to find.
Yahoo's explanation is that the recovered emails were copies created by the email service's "auto-save" feature, which saves data in case of a loss of connectivity, for example. The company has filed several declarations from a number of its staff, but the defense said some of those contradicted each other, and it wants more information.
The defense requested a half-day deposition and a wide range of documents related to the design of Yahoo's email and retention system, a copy of the retention software source code, and instruction manuals for the peripheral equipment that was used to retrieve the emails.
But Yahoo has described the request as a "fishing expedition" and "unreasonably intrusive." Judge James agreed somewhat, and instead ordered the company to release a more limited list of documents, and prepare a witness to talk about specific topics, relating only to the email account in question. If necessary, the documents will be filed under a protective order.
Yahoo has until August 31 to produce a witness for the deposition and provide any documents.
Thumber from the defense told Motherboard in an email, "We are very pleased with the Judge's decision who was able to see the obvious contradictions and problems with Yahoo's explanations. Once we obtain the material, the same will be reviewed in order to advance our UK appeal."