Just before 1:55 a.m., an argument inside the bus escalated when one of the men pulled out a gun and shot two other men, Chicago police said.
A 22-year-old man was shot in the head, and he was in critical condition at Illinois Masonic Medical Center. A 33-year-old man was shot in the chest, and his condition was stabilized at Northwestern Memorial Hospital.
A third man, who is a person of interest in the shooting, had a gunshot wound to his left hand, police said. The man, who is 31, was taken in good condition to Illinois Masonic Medical Center. There weren't any other reported injuries.
The men were among 19 people wounded, four fatally, in shootings from Saturday to Sunday.
People who had been inside the bus with the men huddled inside the lobby of a condominium building in the 3900 block of North Lake Shore Drive. Women comforted one woman who was sitting down crying. Another woman noticed there was blood on her own clothes.
People inside the lobby declined to comment on what happened inside the bus.