Officer Mulrooney succumbed to a gunshot wound received one month earlier when he interrupted a robbery in progress at Joe and Pete's Tavern, a local tavern, at Ninth and Mullanphy. Officer Mulrooney and his partner noticed a car with an open door in-front of the tavern. The officers closed the door and then went inside to tell the owner. As they entered two suspects were in the process of robbing the place and yelled at the officers to stop. Officer Mulrooney ran at the suspect as he opened his raincoat to get his revolver.

During the ensuing the struggle the suspect's shotgun discharged, striking Officer Mulrooney in the upper thigh. Both suspects fled but were later apprehended. Officer Mulrooney struggled for his life for over one month before contracting blood poisoning, causing his death.

Officer Mulrooney was a WWI Navy veteran and was with the agency for 36 years. He was survived by his wife, Rose, and their children. Interred: Calvary Cemetery.