Heidi McGovern, 27, who was holding her 2-year-old son, Enoch, left a suicide note, according to Stephen McCausland, spokesman for the Maine Department of Public Safety.
"State police have concluded that this is likely an apparent suicide," McCausland said. "The woman, holding her 2-year-old son, stepped out from the breakdown lane into the travel lane in front of a tanker truck. She was killed instantly.
"The impact took the child out of the woman's arms and away from the truck. He came to rest in the passing lane. He is currently being treated for multiple injuries at Eastern Maine Medical," he said.
McGovern had parked her car in the breakdown lane of the highway, about four miles north of Lincoln, and was struck at around 4:30 p.m. by an Irving Oil truck driven by a Bucksport man.
The boy was taken to Northern Light Eastern Maine Medical Center in Bangor, where he was listed in critical condition Friday afternoon, according to a hospital spokeswoman.
McGovern is survived by an infant and the father of the two children. He remained at the hospital on Friday.