BBC Worldwide and Channel One Russia investigated the leak of the season four finale of the BBC One/Masterpiece drama series “Sherlock” after a Russian version of the program was leaked online. Deadline.com reports that the leak occurred ahead of last Sunday’s U.K. and U.S. airings of the show.

The investigation “determined it was an isolated and unintentional security lapse on the part of a Channel One employee,” Deadline reports.

Channel One issued a statement saying: “We regret to confirm that one of our employees non-maliciously broke our strict security protocols. As a result of this gross dereliction of duty, the file leaked online. We would like to reassure our partners and viewers, that as a result of this incident, we have completely revised our content security procedures to ensure that this does not happen again and we apologize to Hartswood Films, BBC Worldwide and Sherlock fans everywhere who have been impacted by this leak.”