Mercy-Fitzgerald Hospital in Darby, Penn., was on lockdown Thursday afternoon after a shooting incident which left one dead and two injured. The Associated Press reports:

A doctor grazed by gunfire from a patient who had entered his office in a suburban hospital’s psychiatric unit stopped him by returning fire with his own gun and injuring him, 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.