Cook County prosecutors have revealed the details of a stabbing on Chicago’s L train last Saturday according to Chicago Sun-Times. Tony Polk killed 54-year-old Troy Johnson after Johnson attempted to protect another commuter from an attack.Polk reportedly entered the train with a female associate and soon, after sitting down, lunged at the woman, attempting to grab a folding knife that had been tucked in her belt. After a struggle, the woman broke free and fled the scene. Meanwhile, Polk used the knife to take a nearby passenger hostage. Johnson tried to help the man and, as a result, Polk and Johnson fell from the car onto the platform. Polk then stabbed him to death.

The woman suffered a cut on her hand and fled to a nearby restaurant for help. There, she called Polk and he mentioned that he “beat up” Johnson. Polk then met up with the woman and returned the knife, which had been cleaned of blood. The woman went to police after seeing photos and videos of herself on TV and realizing the extent of the situation.

The police are currently in possession of surveillance footage from the train and Polk is being held for murder in the first degree.