DARBY, Pa. (AP) — A psychiatrist who was grazed by gunfire from a patient at a hospital on Thursday helped stop the patient by apparently using his own weapon to shoot and wound him, but not before a caseworker was killed, authorities said.

The patient opened fire after entering the doctor's office at Mercy Fitzgerald Hospital with the caseworker, District Attorney Jack Whelan said. Witnesses reported hearing yelling before the gunshots.

Several hours after the shooting, investigators had only limited information about what happened inside the closed office but believe the psychiatrist, "from all accounts, would have acted in self-defense," Whelan said.

The doctor, who suffered a wound to his head, "faced a situation where his life was in jeopardy," Whelan said. He was expected to be interviewed by detectives late Thursday.

The hospital has a policy barring anyone except on-duty law enforcement officers from carrying weapons anywhere on its campus, a spokeswoman for the Mercy Health System said.

But Yeadon Police Chief Donald Molineux said that "without a doubt, I believe the doctor saved lives."

"Without that firearm, this guy (the patient) could have went out in the hallway and just walked down the offices until he ran out of ammunition," the chief said.

The dead caseworker was identified only as a 53-year-old Philadelphia woman. Police were working to notify her relatives late Thursday.

Two guns were recovered, Whelan said. Authorities said the motive for the shooting was unknown.

The patient, who was critically injured and was being treated, was identified as Richard Plotts, an Upper Darby resident in his mid-30s.

The prosecutor said Plotts had been involved in previous incidents with staff, but he did not know their nature. He also said he did not know if that is why the doctor had a gun or if the doctor would have been required to have a permit.

After the door of the office was closed, staff members heard loud arguing inside, opened the door and noticed the patient had a gun pointed at the doctor, Whelan said. They closed the door and dialed 911. Gunshots were heard a short time later, just before 2:30 p.m.

Plotts emerged from the office, and another doctor and a caseworker helped wrestle him to the floor of the hallway and grabbed his weapon, Whelan said. By that point he had already been severely wounded from several shots, Whelan said.

"They acted vigilantly. They acted bravely," he said.

The exchange of gunfire occurred on the third floor of the Wellness Center at Mercy Fitzgerald Hospital, a 204-bed community teaching hospital just southwest of Philadelphia.

Authorities said there are no surveillance cameras in the doctor's office or the waiting area outside. They also said the center had no metal detectors.

"Do you evaluate that now ... in light of this incident to make sure people are safe, especially in what can be a dangerous environment?" Whelan asked.

Patients waiting in the first-floor lobby reported a tense scene when police arrived and ordered everyone out. Most of the patients were elderly.

"I dozed off, and I heard the cop shouting, 'Come on, come on, get out!'" said Millicent Russell, 73, of Lansdowne, who was waiting for a 3 p.m. appointment. "There were people with walkers and canes and stuff. All these cops were outside running here and there with these guns."