As she walked to school on a cold March morning, Strothers approached her from behind and shot her three times in the back of the head. Chang was rushed to the emergency room, but it was too late.
"My mom said, 'Scott shot Penny.' And I was like, 'Is she okay?' My mom was like, 'No, she's not okay,'" says Penny's sister, Joanne. "Oh my gosh. Those words, 'She's not okay.' 'Well, how bad off is she?' She said, 'She's dead.'"
It was unbelievable, says Sean, who was so shocked that his best friend would kill his sister.
Six months after the shooting, Strothers pleaded guilty to murder. He's now serving a sentence of 23 years to life.
But for the Chang family, Strothers isn't the only guilty party.
Prior to Penny's death, Strothers was receiving psychiatric care -- first as a patient at the Cleveland Clinic, and then, when he was released, from a private therapist.
The Chang family believes these professionals could and should have stopped Strothers from killing their daughter. Now they're suing them for $20 million.
"The case is pursued as a wrongful death case," says Paul Kaufman, the Chang's attorney. "It's based on concepts of what the public would generally call medical negligence or medical malpractice."
But Kaufman admits it won't be easy trying to convince a jury that psychiatrists and therapists share a responsibility for Penny's death. He believes the biggest failing on the part of the Cleveland Clinic was discharging him when they did.
Everyone agrees that when Strothers first arrived at the Cleveland Clinic -- after stalking Penny and facing charges of harassment -- he was a severely troubled young man.
"In many ways, he looked quite ordinary, although the concerns were extraordinary about him," remembers forensic psychiatrist Kathleen Quinn.
Quinn says Strothers exhibited a number of behaviors that made him a significant risk for violence, including "severe anger," "homicidal thoughts," and a desire to "obtain a weapon." She says that resulted in immediate hospitalization and having commitment papers drawn up.
What's more, Strothers made no secret of his desire to kill Penny in his journal. But Strothers appeared to improve, and, after just five weeks of treatment, he was discharged even though hospital records show doctors still had some concerns.
"Two days before he was discharged, they're having a meeting and nobody in the meeting felt comfortable with this man," says Kaufman. "They didn't believe him. And they doubted his credibility. He was showing no remorse and he was having no emotional reaction. I don't see how anybody could think that this young man was ready for discharge."
George Tesar, the attending psychiatrist who authorized Strothers' release, also had some concerns about his veracity. "They weren't consistent concerns from day to day," he says. "And by the end of the evaluation, I was quite convinced he was telling us the truth as he felt it then."
In an exclusive interview with 48 Hours Investigates, Tesar says he released Strothers only after seeing great improvement in his patient.
"He no longer made those statements about wanting to kill her or feeling any compulsion to kill her," says Tesar, who said the homicidal thoughts expressed in Strother's journal were all in the past tense. "He was feeling better ... He was not conveying the same kind of impression at end of hospitalization."
The family believes the Cleveland Clinic knew Strothers was still a danger and let him out anyway. But Tesar says that if Strothers indeed had been a danger, they never would have discharged him.
Attorney Jim Malone is defending the Cleveland Clinic. "This man's assessment at discharge was correct because nothing happened when he was discharged," says Malone.
He says expecting psychiatrists to predict violence that far into the future is expecting too much: "The Cleveland Clinic has every medical device to benefit humankind. The one thing they don't have is a crystal ball that works."
"Our clinical experience suggests that we're pretty good at predicting violence in the short term -- 24, 48, 72 hours after an evaluation," adds Tesar. "Beyond that, nobody's very good at predicting violence."
If the Cleveland Clinic couldn't predict that Strothers would become a killer, what about the private therapist who picked up his case and was seeing him regularly -- right up until the day he bought a gun and killed Penny?
Strothers met with Raina Krell regularly for three and a half months. On March 12, 1999, Strothers bought a gun, took it to a shooting range and practiced firing it.
Krell refused to talk with 48 Hours Investigates. But she told the court that Strothers never made any threats of any kind, or showed any evidence of present dangerousness.
"He had talked about comments of regret, feeling personally responsible," says Krell in court. "He demonstrated an interest in work."
"I think she looked at him as sort of the jilted boyfriend and never really got below the surface," says Kaufman.
Is it possible, however, that Strothers was such a good actor that he fooled the doctors as well? "It's possible," says Kaufman. "But the problem with that is these doctors are the experts."