NOTHING WAS GOING to stop Richard Plotts from carrying a gun.

Not the law, not his felony record, not his questionable mental health and certainly not a sign.

Plotts, who shot and killed his caseworker and shot his doctor at a Delaware County hospital campus Thursday, may have done so because he was offended by the hospital's policy against guns, Delaware County District Attorney Jack Whelan said.

"There's evidence that he took offense to the issue that there were signs posted at Mercy Fitzgerald Health System indicating that it was a gun-free zone," Whelan said. "That's the only motive we have been able to determine at this point in time . . . he was upset about that policy."

And, if not for an armed doctor and two other brave staffers who subdued him, Plotts may have claimed many more lives with the 39 extra bullets he had in his pocket, according to prosecutors.

"We believe he was there and he was going to reload that revolver and continue to fire and continue to kill," Whelan said.

The incident that shook Mercy Fitzgerald Hospital's campus, which spans both Darby and Yeadon boroughs, began about 1:30 p.m. when Plotts, 49, showed up an hour early to his appointment with psychiatrist Lee Silverman at the Sister Marie Lenahan Mercy Wellness Center, across the street from the main hospital.

Silverman called Plotts' caseworker, Theresa Hunt, 53, of Philadelphia, to join the two in his office, which was common practice, Whelan said.

Plotts was allegedly agitated during the meeting and would not sit. After a brief rant, he pulled a .32-caliber revolver from his waistband, Whelan said.

"What happened next was horrible," he said. "Mr. Plotts took the gun, pointed it directly at Ms. [Hunt] and fired into her head. . . . That bullet went right into her brain and we believe it was instant death at that point."

A "panic-stricken" Silverman crouched behind his chair, reached into his pocket and pulled out a .32-caliber semiautomatic handgun, for which he had a concealed-carry license.

"The doctor indicated that he pointed his gun toward Mr. Plotts and fired until it was empty, not knowing whether he hit Plotts," Whelan said.

Three of Silverman's shots hit Plotts - two in the torso and one in the arm - but it didn't seem to stop him, Whelan said. Plotts was able to get off at least one shot at the doctor, who was hit in the thumb and suffered a graze wound to his face as he tried to cover it with his hand, police said.

A caseworker who was waiting in the office's lobby, John D'Alanzo, climbed through a lobby window, went into the room and wrestled Plotts to the floor with help from another doctor, Jeffrey Dekret, who secured Plotts' firearm.

Whelan called the actions of the three men "heroic."

Hunt was pronounced dead on the scene. Silverman and Plotts were taken to the Hospital of the University of Pennsylvania. Silverman was treated for his injuries and released. Plotts remains hospitalized and under sedation, although he is expected to recover.

When he does, he will be charged with murder, attempted murder and related offenses, Whelan said.

Plotts, who is a convicted felon, is prohibited by law from owning a weapon. Whelan said investigators are still tracking where the gun Plotts used came from and how he obtained it.

Plotts' criminal history dates back to 1990, when he was twice arrested in Philadelphia for carrying a firearm without a license. In 1992 he was convicted of simple assault and forgery, but it was a 1996 conviction for a federal bank robbery that made him ineligible to own a weapon ever again, according to prosecutors.

But that didn't stop Plotts. In 2003, he was convicted of another gun violation in Delaware County.

In 2010 and 2013, Upper Darby police involuntary committed Plotts for mental-health issues and suicide attempts.

Plotts was so violent that he'd even been banned from the Life Center of Eastern Delaware County, an emergency homeless shelter, according to Upper Darby Police Superintendent Michael Chitwood.

"It's tough when you're barred from the shelters," Chitwood said. "They'll take anybody."

Despite his mental-health history, which authorities have not detailed, Plotts was "oriented to reality" during Thursday's shooting, Whelan said.

"I believe that he understood right from wrong and I don't believe that he was suffering from a mental illness that would prevent him from understanding what he was doing," Whelan said.

As for Silverman, Whelan said that he believed it was the doctor's practice to carry a gun for self-protection and that he was in compliance with all state laws.

A Mercy Fitzgerald Health Systems spokesman said Thursday it was against hospital policy for anyone other than "on-duty law enforcement" to carry weapons but a subsequent statement from a Mercy spokeswoman yesterday said, "We look forward to Dr. Silverman's return to serving patients at our hospital."

Silverman had been working on-and-off with Plotts over the last 20 years. It's unknown how long Hunt had been Plotts' caseworker.

Hunt's sister, Audrey Jane Hunt, posted this to Facebook yesterday:

"Wow now to go on without you Theresa my heart aches I can't call or visit," she wrote. "She wasn't selfish and made me laugh and I embarrassed her to no end I love you and now you are with Mommy and Gary."