Fairfax Circuit Court Judge Barnard F. Jennings, saying a prison term was unnecessary, sentenced Nancy L. Kantarian to an indefinite stay in a private mental hospital yesterday for killing her two young daughters in what prosecutors said was an inexplicable frenzy.

"There is no need to punish you," the judge told Kantarian. "You've been punished severely already and will be for the rest of your life."

On Tuesday, Kantarian pleaded guilty to two counts of voluntary manslaughter for stabbing one daughter 32 times and fatally burning the other in a fire she set at the family's $400,000 Great Falls home during what her lawyers called a psychotic episode.

Kantarian, 30, will be incarcerated in a private psychiatric hospital, paid for by her family, until her doctors convince the court she is well enough to be released. Her attorneys said that could be years from now.

Before imposing the sentence, recommended by prosecutor Robert F. Horan Jr. as part of a plea agreement, Jennings asked Kantarian if she had anything to say. She shook her head.

"Well, Mrs. Kantarian," Jennings then said softly, "there isn't any question but that this is one of the most tragic cases to occur in this court in the more than 20 years I've been here. It's tragic for your children, who are dead; it's tragic for you, it's tragic for your parents, for your husband, for anyone who cares about you."

"There is no need to incarcerate you as a deterrent to others," he continued. "And I don't think we really need to protect society from you."

Jennings then sentenced her to the maximum of 10 years in prison on each count, but he suspended the 20 years conditioned on her confinement to a mental hospital. He also imposed five years of probation.

Horan said incarceration in a private mental hospital was the best possible outcome in the case. Psychiatric care will be provided at no cost to the taxpayer.

Kantarian's family will pay the entire cost of her incarceration, as they have since she was moved in August from the county jail to Springwood Psychiatric Institute, a private, $500-a-day mental hospital in Leesburg.

Yesterday, her attorneys said she would remain at Springwood for several months before being moved to an as-yet-undetermined long-term psychiatric hospital. She has been examined by three state and six private psychiatrists, all of whom agree she is suffering from a serious mental illness and needs long-term psychiatric treatment.

The doctors cannot agree, however, about what is wrong with her or what caused her to fly into a rage and kill her two daughters the night of May 23.

Horan said police don't know exactly what happened in the Kantarian home that night, and may never know.

Fire and police officials arriving on the scene found 6-year-old Talia Kantarian dead of multiple stab wounds in a pool of blood at the foot of a stairway and found 5-year-old Jamie Lee Kantarian burned to death in the girls' still-blazing upstairs bedroom.

Kantarian, at a neighbor's house, repeatedly said she stabbed Talia and set the house on fire. During a rambling, nearly incoherent conversation with a police officer, she said she had become angry at the girls because they were building a playhouse of blankets instead of going to sleep.

Kantarian's husband, a Washington lawyer, was in Denver on business at the time.

Kantarian's psychiatric health was considered so delicate that she was excused from attending her own preliminary hearing. Her attorneys said listening to testimony would set back her treatment for months.

The killings shattered what friends said were the story-book lives of Kantarian and her husband, Harry K. Kantarian.

Mrs. Kantarian comes from a wealthy Rochester, N.Y., family. Her father, John E. Heselden, is deputy chairman of Gannett Co., the Arlington-based media conglomerate that publishes USA Today.

Louis Koutoulakos, one of Kantarian's attorneys, called Kantarian "a very fine young lady" who was the victim of a psychotic episode. "Who knows, but for the grace of God there goes any one of us," he said.