This video is from the 2009 National Geographic documentary The Code Breakers. Although the documentary deals with several stories related to cryptography, I've set the video to start at the beginning the Zodiac Killer segment. The segment is roughly 8 minutes long. There are a few factual errors, but nothing too bad. The piece includes interviews with Dr. David Kahn (the man wrote The Codebreakers - The Story of Secret Writing back in 1967) and Professor Ryan Garlick (the University of North Texas computer science professor who is actively trying to solve the 340).
Of course, if you're generally interested in the subject matter, you may find the entire documentary worth watching.