The verdict means that Howorth will be committed to a mental hospital until doctors determine that he does not pose a threat to himself or others.
Eventually it could lead to his release, and even to his inheriting half of his parents' estate, according to District Attorney Robert Steinberg.
Minutes after the ruling, Judge William Ford ordered deputies to take Howorth to Norristown State Hospital for a 90-day evaluation.
Howorth smiled as the verdict was read. One of his public defenders wept, as did several jurors. After the verdict, the jurors left the courthouse in unmarked cars and did not talk to reporters. Their decision followed four days of deliberation and two earlier announcements that they were deadlocked.
"Obviously we did something wrong," said Douglas Reichley, the deputy Lehigh County district attorney who prosecuted the case and argued for a verdict of first-degree murder.
Steinberg said the jurors might have been sympathetic to Jeffrey Howorth
because of his age, adding: "We have to accept the verdict, but I don't respect the verdict."
Steinberg said he believed that Howorth's mental illness falls short even of the lesser standard for the verdict of "guilty but mentally ill," under which Howorth would have gone to prison if he were successfully treated first in a mental hospital. That verdict was an option that the jury rejected.
"I don't think the jury really understands what kind of insanity you have to have to fit the (insanity) plea," he said.
"Five to 10 percent of the population suffers from depression," Steinberg added. "Five to 10 percent of the population are not going out and committing murder."
Public defender Dennis Charles said Howorth was just beginning to confront the fact of his parents' deaths. "People have told me that he cries himself to sleep at night in prison," Charles said.
Howorth's mental state before, during and after the slayings was the focus of most of his two-week trial.
Reichley sought to portray Howorth as a cold-blooded killer who ambushed his father, George, 46, and mother, Susan, 48, in the Howorths' Lower Macungie Township home on March 2.
Bolstered by expert witnesses, Reichley argued that Howorth hated his parents for making him feel like a failure in school, especially for being unable to match the success of his older brother, Stephen, 20, a student at Pennsylvania State University, who found his parents' bodies.
Howorth fired a total of 14 shots during the slayings, at one point chasing his bleeding and frantic mother back inside the house from the garage.
Reichley said that as Howorth paused while shooting his mother, Susan Howorth screamed: "Why? Why?" Howorth's only answer, Reichley said, was to calmly put another clip into his .22-caliber rifle, pull back the bolt, and fire again.
Prosecutors said Howorth's actions after the shootings - washing his mother's blood off her car keys, locking the door to the house, and fleeing in her car - were further evidence that he was sane.
Prosecutors also introduced a note that linked the killings with the slayings of another Allentown-area couple three days earlier, in which their skinhead sons, Bryan and David Freeman, are accused.
Seeing news reports about those killings "was a liberating act for Jeffrey Howorth," Reichley told the jury in his opening statement.
Howorth's rambling note had said, in part, that the Freeman youths "were cool, they killed their parents. I would be rough if I did that."
Defense lawyer Charles put on expert witnesses who countered that Howorth suffered from severe depression and brain damage. He said Howorth had lived under the shadow of undiagnosed mental illness.
"Jeffrey Howorth held the .22-caliber rifle that killed his parents," Charles contended. "But insanity pulled the trigger."
Howorth's teachers and schoolmates said they had no hint of problems before the slayings. Howorth swam backstroke on his school's swim team, was a Boy Scout, and attended church regularly and voluntarily.
No members of the Howorth family were in court yesterday, though prosecutors and defense attorneys said Stephen Howorth had been in touch through lawyers.
Charles said he had advised Jeffrey Howorth not to talk to his brother during the trial, to protect Stephen Howorth from further testifying.
"His brother has always been his best friend," Charles said.