Two police officers were shot in southwestern Pennsylvania and one died early on Thursday, local media reported.
Police told residents to remain in their homes as they searched for the gunman in the town of Canonsburg, about 25 miles (40 km) southwest of Pittsburgh.
There were no immediate details on the other officer's condition or what led to the attack at about 4 a.m. local time.
One officer had been flown to a hospital in Pittsburgh and the other to a hospital in Canonsburg, WPXI.com reported.
Authorities said schools would open two hours later than usual as the investigation continued. Area roads were closed and residents told to stay indoors.
Police officials were not immediately available for comment.