BOSTON -- A Bellingham woman intentionally walked onto Interstate 495 while carrying her niece and nephew before they were hit and killed by oncoming cars last week, Middlesex District Attorney Gerry Leone said Friday.

Marcelle Thibault, 39, of Bellingham, and her twin sister's children, Kaleigh Lambert, 5, and Shane Lambert, 4, of Brentwood, N.H., were struck and killed by two cars on Interstate 495 North in Lowell on Jan. 11.

"Based on our investigation, we believe that Ms. Thibault took her niece and nephew and walked with them into oncoming traffic on Route 495, tragically resulting in the deaths of the two young children while taking her own life in the process," Leone said in a statement. "The facts of this matter make it a truly heart-wrenching case, with troubling and very difficult findings."

Investigators said according to witnesses, Thibault was driving her 2003 Lincoln LS southbound on I-495 when she turned sharply, drove across the median strip, crossed oncoming traffic in the northbound lanes, and then began driving against traffic in the breakdown lane of the highway.

The Middlesex District Attorney's Office said Thibault eventually stopped the car and removed both her and the children's clothes. She then took the children in her arms and walked into the roadway where they were struck and killed.

"Marci got out of the car with her niece and her nephew. The clothes were taken off both the niece and the nephew; her own clothes were taken off as well. They were placed on the side of the road, and Marci took both her niece and nephew into traffic at approximately 9 o'clock. There was a lot of traffic on 495. The motor vehicle collisions that occurred killed all three of them," Leone said.

Thibault, a stay-at-home mother with two teenaged children of her own, had a brief history of mental illness, Leone said.

"What is absolutely clear is that the brief history of mental illness for Marci Thibault was the cause of what occurred that night and is the reason why Marci, Shane and Kaleigh are dead," Leone said.

Leone would not offer any details about Thibault's mental illness, or how it was treated.

Thibault's family said she was driving the children from their home in New Hampshire to Bellingham for a family sleepover when the incident happened. Leone said the children often visited their aunt for what were called "pirate and princess weekends."

"(Ken and Danielle Lambert) were loving parents. They would not have let their kids go with their aunt unless they were absolutely certain that they were not in any danger," Leone said.

Thibault was taken first to Holy Family Hospital in Methuen and later to Massachusetts General Hospital in Boston, where she died. The children were taken to Lawrence General Hospital where they were both pronounced dead.

Autopsies conducted on the children and Thibault determined that the cause of death was homicide/suicide by blunt force trauma resulting from being hit by two cars, Leone's office said.

"This is a time of terrible tragedy for our families. We love Marci, Kaleigh, and Shane and we miss them very much. 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 victims' family said in a statement.

The children's funeral was held at Saint Theresa's Catholic Church on Thursday in North Reading. Thibault's funeral was held Wednesday at Saint Blaise Church in Bellingham.

The drivers of the cars that struck the victims were not injured and have not been charged.