CSI: Crime Scene Investigation - Season 4 Episode 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.