Story highlights DA spokeswoman: Richard Plotts is charged with first and third degree murder, among other charges

Psychiatrist uses gun he was carrying to shoot suspect, authorities said

A caseworker was killed and two people, including suspect, were wounded in the shooting

A Pennsylvania patient accused of shooting his caseworker to death has been charged with first and third-degree murder, authorities said.

Richard Plotts was arraigned late Saturday at his hospital bed, said spokeswoman Emily Harris of the Delaware County District Attorney's office.

He is also charged with attempted murder, recklessly endangering another, assault and simple assault, and carrying a firearm without a license, Harris said.

On Thursday, Plotts, 49, walked into the psychiatric unit Mercy Fitzgerald Wellness Center in Darby, Pennsylvania, alongside his caseworker Theresa Hunt, 53, according to Delaware County District Attorney Jack Whelan.

The two went into psychiatrist Dr. Lee Silverman's office and an argument erupted, Whelan said.

Hunt shut the door and called police.

Moments later, shots were fired.

Another doctor and caseworker ran into the room and found Hunt wounded, authorities said. The physicians wrestled Plotts to the floor and held him until police came.

A bullet grazed Silverman's head. He was treated and released after investigators interviewed him.

Silverman used a gun he was carrying to shoot Plotts in the arm and torso, authorities said.

Plotts was being treated at the Hospital of the University of Pennsylvania.

It's unclear why Plotts opened fire, authorities said last week.

Whelan told CNN affiliate WPVI, "This could be a much different situation if the perpetrator was allowed to continue with his actions, so I give credit for the staff for intervening."

Two guns were recovered at the scene -- Plotts' and Silverman's. It wasn't certain who shot Hunt but, according to Whelan, investigators believe Silverman shot Plotts in self-defense.