Two police officers were shot in southwestern Pennsylvania early on Thursday, local media reported.
Police told residents to stay inside their houses 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 officers' conditions or what led up to the attack at about 4 a.m. local time.
One of the officers was flown to a hospital in Pittsburgh and the other was taken by ambulance to a hospital in Canonsburg, WPXI.com reported.
Authorities said schools would open two hours later than usual as the investigation continued.
Police officials were not immediately available for comment.