Law & Order: 9x14

Sideshow (I)

When Briscoe and Curtis discover that a high-level federal official found murdered was recently transferred from Baltimore, they re-team with Baltimore homicide detectives. The joint investigation reveals that the victim was involved in an affair with a high-level government official whose career will be destroyed if it becomes public. The independent counsel summons McCoy and Ed Danvers to Washington to demand McCoy reveal his source or be jailed for contempt.

This crossover continues on Homicide: Life on the Street S07E15 Sideshow (II).