Federal investigators say a gun which killed a commander with the Chicago Police Department has been traced to a Milwaukee house. Commander Paul Bauer was killed while chasing a man last month. Federal investigators say the gun was originally purchased legally outside Madison about six years ago. It was traded at a gun club in 2015. Last year, it was sold online to a Milwaukee man. A story in the Chicago Tribune reveals that the man is a convicted killer. Federal agents raided his home two weeks ago. They found body armor and at least 30 guns.