DARBY, Pa. (AP) — A doctor wounded by a patient in a deadly shooting at a suburban hospital campus returned fire and wounded the patient, authorities said.

Investigators believe the doctor had his own gun and acted in self-defense, District Attorney Jack Whelan said.

The patient, who had psychiatric problems, opened fire after entering the doctor's office with a caseworker Thursday afternoon, Whelan said. The caseworker was killed. The doctor appears to have suffered only a graze wound and was expected to be released from treatment later in the day.

Authorities have not publicly identified the gunman or his two victims.

The shootings occurred on the third floor of a wellness center attached to Mercy Fitzgerald Hospital in Darby, just southwest of Philadelphia. The center, which does not have metal detectors, is attached to the hospital by a pedestrian bridge.

Just what led to the shooting was unknown, Whalen said. Two guns were recovered, and the shooting was still being investigated.

"The scene is secure. Everyone is safe," Whelan told a late afternoon news conference.

The 213-bed teaching hospital is part of the Mercy Health System, a large Catholic health care network serving the region.