The 63-year-old officer was the third off-duty Chicago police officer killed in the past two months.
The officer's name was not immediately released; however, the Chicago Tribune said sources reported he had been assigned to watch Mayor Richard Daley's residence during the overnight shift.
Investigators told the Tribune the victim had just returned home on South Evans Avenue around dawn and was outside cleaning his car windshield when three men approached. There was an exchange of gunshots and the suspects all fled.
Details of the altercation were not immediately released and the suspects remained at large.