"We had information to believe there may be very peculiar postings," Whelan said, possibly including some in which Plotts, 49, calls himself a "potential killer." Whelan said the computer had not yet been reviewed.
On Thursday, Plotts, a patient on and off for 20 years at Mercy Fitzgerald Hospital, on the border of Darby Borough and Yeadon, carried a loaded revolver and 39 bullets into his appointment with psychiatrist Lee Silverman at the hospital's Wellness Center.
Plotts was seated next to caseworker Theresa Hunt when he is said to have become agitated and started shouting. After they stood, he pulled out a revolver and fired point-blank at Hunt, 53, authorities have said.
Silverman, 52, dropped behind a desk chair, drew a handgun from his pocket, and fired back, hitting Plotts three times, just before two hospital staffers burst into the room, tackled Plotts, and subdued him.
"If the doctor . . . did not utilize the firearm, he'd be dead today, and I believe that other people in that facility would also be dead," Whelan said Friday.
Silverman, who was treated and released from the hospital for a graze wound to his head, was unavailable for comment.
The Wellness Center - about 200 yards from the main hospital building, across a small street - was closed Friday after the incident. It reopened Saturday morning and closed at noon, its normal time.
Hospital spokeswoman Bernice Ho said in a statement Saturday: "The Mercy Fitzgerald family continues to cope with the shock and sadness. . . . We are grateful for the prayers and outpouring of support from the community."
At a Friday news conference, Whelan said Plotts had expressed anger at least once before at not being allowed to take a gun inside the hospital.
Plotts, a felon, was prohibited from owning a weapon.
His criminal record dates to at least 1990, when he was arrested in Philadelphia and sentenced to probation for carrying an unlicensed gun. His record also includes an 80-month federal sentence for bank robbery.