CAMBRIDGE, Mass. - Marcelle Thibault was a stay-at-home mom so she could take care of her own teenage children. She volunteered to organize activities for teens at her church.

She often threw parties for her young nieces and nephews, like the "pirates and princesses" sleep-over she picked up her twin sister's only children for Jan. 11.

That night, driving on Interstate 495 in Lowell with 5-year-old Kaleigh Lambert and her 4-year-old brother, Shane, Thibault turned her car sharply, crossed the median and began driving against traffic in the breakdown lane.

Middlesex District Attorney Gerry Leone said Friday that Thibault stopped on the right side of the road, got out of the car, removed her clothes and undressed the children.

She took them in her arms and walked onto the highway. All three were killed when two cars hit them.

Leone blamed mental illness, but said there's no way to know exactly what was going through the 39-year-old's mind.

"There are not many other scenarios I can think of that are as tragic as this one," Leone said. "It is beyond belief, it is unimaginable, it is unspeakable, and it was a horrible tragedy."

Leone described Ken and Danielle Lambert of Brentwood, N.H., as loving parents and said they would not have allowed their children to go with their aunt had they not been certain they would be safe.

A neighbor of the couple described them as caring and conscientious. Their children, when they played outside with chalk, would draw a line halfway down the driveway to mark how close they could go to the street, he said.

Murder-suicide 'inconceivable'
Leone said Thibault of Bellingham, had received treatment for mental illness. Paul Young, a spokesman for the Lambert family and friend from St. Michael's Parish in Exeter, N.H., did not know the details, but said it was a "brief, isolated incident in her life" within the past year.

"She appeared to be fully recovered from that, and there was no indication of a relapse," he said.

"I never saw any signs of problems," said the Rev. David Mullen, pastor of St. Brendan's Church in Bellingham, where Thibault was a frequent volunteer. Mullen described her as "very positive and very generous."

Murder-suicide, he said, was "inconceivable."

'Don't believe it's true'
Wally Barnes, 76, said he lived next door to Marci Thibault's mother, Candelora Coady, in Bellingham for 44 years and knew Marci as a child. He also said he never saw any evidence of mental illness.

"I just don't believe it's true," he said. Barnes said he saw Thibault and her mother raking leaves in the yard last fall with several children.

Leone said no drugs or alcohol were involved. "It is far too complicated, too complex to make any real determination about what the cause was, what was in her mind," he said.

A statement from the family released by Leone's office pleaded for prayers and privacy.

"This is a time of terrible tragedy for our families. We love Marci, Kaleigh, and Shane and we miss them very much," the statement said. "We ask everyone to join us in prayer for their souls, and to help us get through this most difficult period of our lives."

The Rev. Marc Drouin of St. Michael's Parish said Ken Lambert spoke at his children's funeral Thursday.

"Ken's words yesterday at the funeral, that he spoke at the end of the Mass, said it so well, that people asked how can we help you, and his response was just, 'Love your families, love your children and be present with them because you never know when that will be the last time that you see them.'"