Callahan's decomposed body was found in the trunk of a car parked at Miami International Airport.
In closing arguments Tuesday, prosecutor Michael Von Zamft told the jury Connolly "decided he would rather be a gangster than an FBI agent."
Connolly rose to FBI stardom in the 1970s and 1980s for cultivating criminal informants in order to dismantle the New England Mafia.
He is serving a 10-year prison term for a 2002 conviction on federal racketeering charges.
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