Sgt. Robert Carpenter, 46, was shot in the head early Sunday while responding to a triggered alarm at a Tucson home, the Arizona Daily Star (Tucson) reported.
Carpenter is in stable but critical condition, said Police Chief Roberto Villasenor.
Other officers at the scene did not see who fired the shot that struck Carpenter, or know where the shot came from, said Sgt. Maria Hawke, a TPD spokeswoman.
The Special Weapons and Tactics team, K-9 units, a police helicopter and patrol units flooded the neighborhood in which the shooting happened Sunday, and SWAT officers went door to door contacting families and searching the neighborhood into the early afternoon.
As of Sunday night, police had no suspects in the shooting, the newspaper said.