S4 E1- Fruit of the Poisonous Tree (1)

While Frank is in Washington to talk to Paul about returning to work, Dan calls to say Vinnie missed his last check-in. Frank goes to Vinnie's house and finds evidence that he was kidnapped. Despite Paul's objections, Frank begins his own investigation, finding out from Carlotta that Vinnie was looking into the death of a priest in El Salvador. Frank follows the trail to Miami and Michael Santana, a disbarred Federal prosecutor, and the last person Vinnie contacted. Santana refuses to help Frank, saying he doesn't know anything about Vinnie's disappearance. Digging deeper, Frank stumbles upon a CIA investigation and is ordered to back off. (source: http://www.plasticcow.com/wiseguy)