A recently paroled armed robber is being held without bond in the stabbing death of a man aboard a Chicago Transit Authority train.

Tony Polk is charged with first-degree murder in the death of 54-year-old Troy Johnson, who authorities said was trying to protect another passenger from a knife-wielding attacker.

During a Wednesday bond hearing, Cook County Assistant State’s Attorney Jim Murphy says the 40-year-old Polk and Johnson were aboard a Red Line train Saturday, July 13 when Johnson tried to help a passenger allegedly being threatened by Polk. During the ensuing fight, Polk allegedly stabbed Johnson several times before fleeing at the Cermak-Chinatown Red Line stop, which is the closest stop to Wintrust Arena, where the Blue Demons play their home games.

A woman seen in surveillance photos came forward and led police to the hidden knife Polk used in the attack, which led to Polk’s arrest Monday, July 15.

Murphy says security cameras also recorded the attack.

It wasn’t immediately known whether Polk had a lawyer.