A 19-year-old man and a 17-year-old girl died at the scene of the shooting reported just before 1 p.m. on Milwaukee's west side, according to Flynn and the Milwaukee County medical examiner's office.
The two teens, along with a 21-year-old man, got into an argument with the 39-year-old maintenance man in a stairwell in a common area of the building, Flynn said.
One of the three beat the maintenance man while he was being held, the Milwaukee Journal Sentinel reported. The maintenance man then pulled a gun and shot the two teens.
Flynn said the maintenance man was taken to a hospital with non-life-threatening injuries. The 21-year-old man was arrested.
Witnesses were being interviewed Wednesday afternoon, including one who caught the beating and the shooting on video, Flynn said.
The two teens did not live in the building but knew people who lived there, the chief said.
There was no indication that a robbery was taking place, and authorities will determine if charges will be filed as a result of the shooting, Flynn said.