Michael Winter

USA TODAY

A psychiatric outpatient opened fire Thursday inside a psychiatrist's office at a hospital near Philadelphia, killing his caseworker and slightly wounding the doctor, who shot the gunman with his personal firearm, authorities said.

The suspect, Richard Plotts, of Upper Darby, Pa., was reported in critical condition after the shooting at 2:20 p.m. in an office at the Mercy Wellness Center of Mercy Fitzgerald Hospital in Darby, Delaware County District Attorney Jack Whelan said at an evening news conference.

The unidentified 52-year-old doctor shot Plotts three times and suffered a graze wound when the suspect returned fire, Whelan said at an evening news conference. Two guns were recovered.

Another doctor and caseworker tackled Plotts in a hallway and held him until police arrived.

Whelan said Plotts, who has a history of unspecified psychiatric problems, and his caseworker arrived at the doctor's third-floor office about 2 p.m., Whalen said. Soon after, another staffer heard a loud argument and opened the door to find the suspect pointing a gun at the doctor. The worker then closed the door and call 911.

Minutes later, gunfire erupted.

"We're not exactly sure what had occurred and what might have precipitated this particular incident," Whelan said at an afternoon news briefing. "We do know the psychiatric connection, we have that concern. However, we don't know if there was another dispute that occurred inside that room that led to these shots being fired."

The identity of the slain 53-year-old caseworker was not released, pending notification of her family. Whalen did specify the nature or number of her wounds.

Plotts, described as being in his mid-30s, was in surgery at the Hospital at the University of Pennsylvania. He was shot twice in the torso and once in an arm.

The wellness center was evacuated and the campus locked down briefly as SWAT officers checked to be sure there was not another gunman.

Neither the hospital nor clinic, which are connected by a pedestrian bridge, has metal detectors.

A sign tells visitors to the wellness center to check weapons at the front, a medical technician toldThe Philadelphia Inquirer.

Hospital policy allows only on-duty law enforcement officers to carry weapons on campus, a Mercy Health System spokeswoman told the Associated Press.

Hospital spokeswoman Bernice Ho was not available Thursday night to clarify the policy or comment on the doctor's actions.

"This is a tragic event for our hospital and for our community, and we are praying for the safety of the victims," the hospital said in a statement. "We are grateful for the efforts of the emergency responders, and we are working with the Delaware County Police Departments to understand fully the details of the event and to do all we can to assist those affected."

The wellness center remained closed Thursday night and Mercy Fitzgerald was diverting patients to other hospitals.

Hospital spokeswoman Bernice Ho was not available Thursday evening.