The prisoner's brother was killed in the attack.
Domenico Cutri, 32, remained at large. He was under watch by guards when three gunmen attacked the prison van near Milan and freed him, the Italian news agency ANSA reported.
Antonio Cutri, the prisoner's brother, was shot by guards during the exchange. The group dumped him at a Milan hospital, where he was declared dead.
Domenico Cutri was convicted of killing a Polish man in northern Italy in 2006 because the victim was "bothering" his girlfriend.
It was unclear whether any of the guards were injured in the attack.