Stuart L. Kreiner, the frail and bespectacled 20-year-old who four years ago pleaded guilty to murdering three Anne Arundel County school girls, committed suicide yesterday at Patuxent Institution in Jessup by hanging himself with a bedsheet attached to bars of his prison cell.

Kreiner, who was serving a life sentence at the psychiatric institution and would have been eligible for parole in 1990, left behind a note to his family, state police and prison officials said. Contents of the note were not disclosed.

Richard Hogan, father of two of Kreiner's victims, said tears came to his eyes when he learned of the suicide, which he believes was "grossly unnecessary. If there had been a trial, if everything had come out, then Stuart would not have had to live with this inside himself," Hogan said.

There was no trial. The case ended in a plea-bargain agreement that left many questions unanswered about what occurred the night of Oct. 9, 1977, when 10-year-old Debbie Hogan, her sister Terry, 8, and their 8-year-old playmate, Ann Brzeszkiewicz, were stabbed to death. Their bodies were found the next morning in the wooded stream near the middle-class Glen Burnie subdivision where the victims and their killer lived.

The case became one of the area's most notorious slayings when police arrested Kreiner, then 16, who was described as a quiet youth and a church choirboy whose strict adherence to his parents' rigid family rules made him an outcast among his peers.

Kreiner pleaded guilty to the slayings in October 1978. As part of the plea-bargaining arrangement, the youth was sent to Patuxent, the state facility offering a wide range of psychological treatment for prisoners deemed capable of rehabilitation.

Because of his youth and his good behavior, Kreiner was transferred two years ago to the prison area with the least security. The solid metal door of his single cell was left unlocked and guards visited his hallway only periodically at night.

Prison officials were puzzled by the suicide. They said Kreiner had shown no signs of depression. He had attended a mandatory group counseling session on Tuesday. "It doesn't seem like there was anything wrong," said prison spokesman Beverly Marable. "But obviously there was." The last suicide in Patuxent was in August 1981, she said.

A specialist in forensic psychiatry said yesterday that suicides like Kreiner's must be viewed in light of the "basic personality" that led to his original "violent act." Dr. Lawrence Brain, director of forensic psychiatry for Professional Associates in Bethesda, said that frequently there is a sense of loneliness, helplessness and emptiness among people like Kreiner. "Suicide is part and parcel of these people. It is always an option. It is not uncommon for them to kill themselves."

Their crimes may, in fact, be "almost an attempt to get society to kill them," said Brain, who has never been involved in Kreiner's case.

Kreiner's parents could not be reached for comment. A family friend said they were "devastated" by the suicide.

Patuxent administrative officer Carl Schlaich would not discuss Kreiner's most recent conversations with prison psychiatrists, but did say, "There was no warning. There was no reason for this." Prison officials said Kreiner had never before attempted suicide.

The news of Kreiner's death "spread like wildfire" through the Southgate community, where some people still were bracing themselves for Kreiner's eventual release, according to Bruce Strazza, who was president of the Southgate community association at the time of the murders. Emotions, according to Strazza, "were mixed. Some people say, 'Well, he almost deserved what he got.' Others were saddened by it."

Kreiner's sentence to Patuxent prompted a statewide outcry led by Hogan and the Southgate residents because they thought the prison's liberal parole provisions could result in Kreiner's release after one year, if he was deemed psychologically fit. The prosecutor in the case, former state's attorney Warren B. Duckett, Jr., and others called for reforms in Patuxent's policy of releasing inmates into community centers once a staff of psychiatrists determined they were ready to reenter society.

This year, the Maryland General Assembly approved legislation requiring the governor to approve all releases and paroles from Patuxent.

Kreiner's suicide does not close the case for Ingrid Hogan and her husband, who said they relive the tragedy daily. "We know what they Kreiner's parents must be going through. We went through that. They will experience the same trauma we have felt. We know what they will be going through the rest of their lives."