Police say the sergeant and another officer went to the off-duty officer's home late Thursday after receiving a report that he may be suicidal.
The off-duty officer eventually put a gun to his head, causing the sergeant to shoot him in the leg. But, as the off-duty officer fell to the ground, he took his own life. He was pronounced dead at his home.
The sergeant is on administrative leave. The shooting is being reviewed by the Milwaukee County district attorney's office.