In May 2004, a Moscow military court sentenced Trepashkin to four years in a penal colony on charges on disclosing state secrets. A civil court in the Urals city of Nizhnii Tagil late last month released Trepashkin on parole.
Trepashkin has said his case is politically motivated.
A lawyer by training and a retired FSB officer, Trepashkin conducted an investigation into the 1999 Moscow apartment bombings that Russia cited to launch its second military campaign in Chechnya.
Russian authorities blamed Chechen separatists for the deadly blasts, but Trepashkin has suggested FSB agents may have been involved in the attack.