Authorities said they believed Silverman shot Plotts three times in the torso and arm. Silverman was expected to be released from the hospital Thursday night and recover fully.
Plotts, of Upper Darby, who appeared to have a long criminal record, was undergoing surgery Thursday night at the Hospital of the University of Pennsylvania. If he survives, he will be charged Friday with murder, Delaware County District Attorney Jack Whelan said.
It was not clear why Silverman, a doctor for nearly 25 years, was armed at the office. Bernice Ho, a spokeswoman for Mercy Fitzgerald, said Thursday it was against hospital policy for anyone other than security guards to carry weapons.
Donald Molineux, chief of the Yeadon Police Department, said that if Silverman returned fire and wounded Plotts, he "without a doubt saved lives."
Mercy Fitzgerald Hospital called the incident "a tragic event for our hospital and for our community," and said its community was praying for the victims.
"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," hospital officials said in a statement.
Founded in 1933 by the Archdiocese of Philadelphia, Mercy Fitzgerald is a 213-bed hospital that serves more than 186,000 patients each year.
Why Plotts was there was unclear. Whelan said Plotts has a history of psychiatric problems.
Court records show that a Delaware County man matching his name and age - authorities said Plotts was in his mid- to late 40s - was sentenced in 1996 to more than seven years in prison for robbing a bank in Wilmington. Other court records show that an Upper Darby man with the same name and age has been arrested numerous times over the last 30 years for assault, drugs, weapons possession, and other offenses.
On Thursday, Whelan described how the meeting among Plotts, Silverman, and Hunt abruptly took a violent turn.
Plotts and Hunt went to Silverman's third-floor office shortly before 2:30 p.m., Whelan said. Plotts was apparently armed, Whelan said, and people near the room soon heard shouting.
Concerned, a hospital employee "actually opened the door, saw him pointing a gun at the doctor," Whelan said. The worker shut the door quietly and immediately called 911.
Plotts then opened fire. According to Whelan, he shot Hunt two times in the face. The psychiatrist then ducked under his desk, retrieved his gun, and came up shooting, striking Plotts three times.
Staff members rushed toward the scene. Plotts ended up in the hallway, and another caseworker and a doctor tackled and pinned him, Whelan said.
As police arrived, the building was being evacuated and placed in a lockdown. Patients and doctors streamed onto the lawn and driveways surrounding the building, which contains offices for outpatient services, including pediatrics, cardiology, pulmonary care, and psychiatry.
Allen Williams of Upper Darby was handing over his identification and health card for his X-Ray appointment when police officers rushed into the lobby.
"They came in with guns drawn," he said. "It was just a shock to me."
Alfred Williams said he had just left a doctor's appointment and was waiting for a ride home when, suddenly, swarms of police officers descended on the scene.
"They kept coming," Williams said. "Guys with helmets and automatic weapons kept jumping out of their cars. It was total panic. . . . I saw three people come out in stretchers."
Anna Smith, an ultrasound technician, was on the first floor of the building celebrating a colleague's 60th birthday when police burst in and told everyone to leave through the back door, she said.
"There's a sign on the door that says you have to check your weapons at the front," she said. "But you can't expect every crazy person to do that."
Former neighbors at Plotts' apartment building along West Chester Pike in Upper Darby said Plotts was an uneasy presence in the neighborhood until he moved out sometime in the last year. The tall, solidly built Plotts seemed like he was either "on drugs or heavily medicated," said Bert Garcia.
"He was a big guy," Garcia said. "He could be intimidating."
Once, before he moved out, Garcia found that he had removed some ceiling tiles in the hallway and was messing around with the wiring. Another time, he told Garcia he had stabbed himself in the leg - but Garcia saw no blood.
"You could tell there was something wrong," neighbor Cathy Nickel said. "He needed help."