Nancy Lee Kantarian pleaded guilty yesterday to killing her two young daughters, stabbing one 32 times in a frenzy and striking and then burning the other in a fire at the family's Great Falls home -- crimes that a prosecutor said remain a mystery.

Kantarian, her face puffy, stood with her head bowed and eyes closed in Fairfax County Circuit Court as the two voluntary manslaughter indictments against her were read, responding with a barely audible "guilty" when asked how she pleaded.

"There is no question the killings are the result of some form of mental or emotional sickness," said Fairfax prosecutor Robert F. Horan Jr. He recommended she be committed to a private psychiatric hospital, saying investigators are baffled at what specific motivation may have transformed a woman who had been "a model parent" and "a doting mother" before she became ill.

Kantarian, 30, sobbed openly during most of the 20-minute hearing as testimony was taken from police and fire personnel about the grim scene they found at the family's $400,000 home when they responded to a fire call the night of May 23.

"My mind, my mind just snapped," Kantarian told a police officer in a disjointed conversation 30 minutes after her 6-year-old daughter, Talia, was found in a pool of blood at the foot of a staircase and 5-year-old Jamie was discovered burned to death in a bedroom.

In the statement to officer Michael M. Mack, made public yesterday, Kantarian spoke of her furor over her children's refusal to go to sleep that night. "Jamie was angry. . . . She built. . . . She wouldn't sleep. . . . She piled blankets to make a house, when she should have been sleeping," Kantarian said. "I got mad. . . . Harry was gone. . . . I get so frightened."

At one point, Kantarian described Talia's death. "The knife, there was a knife," she said. "I just picked it up and started stabbing her. She was running towards the fire, she wanted Jamie."

She repeatedly told police that she stabbed Talia and set the house on fire. "I have to tell you one thing," she said. "When I knew Tally was dead, I just tried to set the hallway on fire. I just wanted the whole house to burn down. I was going to die in the fire, too."

Her lawyer-husband, Harry K. Kantarian, who was in Denver on business that night, sat stony-faced through the proceeding. Her father, John E. Heselden, deputy chairman of Gannett Co., a newspaper-publishing firm, was also in the courtroom.

As part of a plea agreement, Horan recommended Kantarian be indefinitely committed to a private psychiatric hospital -- at her family's expense -- instead of being sentenced to prison.

Kantarian, who has a history of mental illness, has been held in the locked ward of the Springwood Psychiatric Institute, a private, $500-a-day hospital in Leesburg, Va., since mid-August.

Jennings deferred sentencing until Friday in order to read the voluminous reports from nearly a dozen state and private psychiatrists who have examined Kantarian since her arrest.

Jennings is not bound by the plea agreement, and could sentence Kantarian to up to 10 years in prison on each manslaughter count.

Kantarian has refused to discuss the details of what happened that night with any of the psychiatrists who have examined her, Horan said. Investigators have only the sketchiest idea of the sequence of events, he added.

He said police do know Kantarian set the house on fire, struck Jamie, chased Talia downstairs, stabbed her, and washed up and changed her clothes before running next door, shouting "Fire! My children! Fire!"

But he said police do not know in what order the events happened, or what triggered them.

Kantarian had been under psychiatric care for three years prior to the killings, according to court records, but did not appear seriously ill.

Kantarian's three attorneys, including a family attorney from Rochester, N.Y., introduced no evidence on her behalf yesterday and said little during the hearing.

Asked if they had any statement, attorney Albert J. Ahern Jr. said: "No, your honor, we think this is a fair disposition which helps the defendant and protects the Commonwealth."

Under the agreement, Kantarian could be released only when psychiatrists find she is neither a danger to herself or to others, and then only after a court hearing.