In US Forensic Expert: UK Defendant Convicted with NSA Email Sweep (Part 2), internet security expert witness writes:
Nonetheless, 83 pages of email drafts covering about 29 weeks were produced as evidence. SOCA (Serious Organized Crime Agency) Senior Investigating Officer, Paul Ashton, told the jury in the drug conspiracy trial that a letter of request (LOR) for assistance was sent to the FBI, which in turn provided the Yahoo! data that the jury was reviewing. Although the email evidence was discussed in open court, multiple reports by BBC News never mention email.
Speaking from the California laboratory of Burgess Forensics, the analyst said, “With Snowden’s releases in the Washington Post and Guardian and the NSA’s subsequent acknowledgement of the formerly secret PRISM program, we now know a means for that Yahoo! data to have surfaced, and at this point PRISM is the only possible known route for it to have come to light.”