CSI: Crime Scene Investigation - Season 4 - Eps 20: Dead Ringer
Grissom, Catherine and Nick investigate the death of a runner during a competition. Meanwhile Sara and Warrick investigate the death of couple in a hotel room, initially thought to be a murder-suicide. Both cases take place during this 'cop' competition , with more than 20.000 attendants (all of them cops). This gives the investigations an interesting turn, as almost all the suspects and all dead people are cops.
About CSI: Crime Scene Investigation

Title: CSI: Crime Scene Investigation
First Air Date: 2000-10-06
Last Air Date: 2015-02-15
Status: Ended
Rating: 7.636/10 (from 1280 votes)
Language: EN
Seasons: 16
Total Episodes: 335
Network: CBS
Genres: Crime, Drama, Mystery
Production Companies: Alliance Atlantis, CBS Productions, CBS Studios
Synopsis
A Las Vegas team of forensic investigators are trained to solve criminal cases by scouring the crime scene, collecting irrefutable evidence and finding the missing pieces that solve the mystery.
Cast

Ted Danson
D.B. Russell

George Eads
Nick Stokes

Jorja Fox
Sara Sidle

Eric Szmanda
Greg Sanders

Robert David Hall
Al Robbins

Wallace Langham
David Hodges

Jon Wellner
Henry Andrews

Elisabeth Harnois
Morgan Brody

Elisabeth Shue
Julie Finlay

David Berman
David Phillips