Law & Order S16E20 - Kingmaker

After undercover cop Dana Baker is murdered, Fontana and Green learn that the man who killed her had discovered her identity after seeing a photograph of her in the paper revealing her as an undercover police officer, not the heroin dealer she was posing as. Investigating the story leads detectives to Eric Lund, one of the workers in Congressman Prescott's office, but Lund seems to have an ironclad alibi after an e-mail he sent comes to light.