A woman with a history of mental illness purposely walked her young niece and nephew into oncoming interstate highway traffic to commit a double-murder suicide, a prosecutor in Massachusetts said Friday. Marcelle Thibault, 39, was driving south on the Interstate 495 highway in Lowell last week when she turned the car sharply, drove across the median and northbound lanes and began driving against traffic in the breakdown lane, authorities said. She then stopped the car, took both children into her arms and walked onto the highway. She and the children were struck by two cars and killed. "There are not many other scenarios I can think of that are as tragic as this one," District Attorney Gerry Leone said. "It is beyond belief, it is unimaginable, it is unspeakable, and it was a horrible tragedy."