The mother who stepped in front of a tractor-trailer on Interstate 95 while holding her 2-year-old son had left suicide notes at her home, Maine State Police said.

Police are investigating the death of 26-year-old Heidi McGovern, of Lee, as a suicide. McGovern was holding her son, Enoch, when she stepped in front of an Irving Oil tanker truck Thursday afternoon.

McGovern was killed instantly and her son was thrown from her arms. The boy landed in the passing lane of the highway and was taken to Eastern Maine Medical Center in Bangor, where he is still being treated for serious injuries, police said.

Stephen McCausland, spokesman for the Maine Department of Public Safety, said McGovern left notes at her home.

The crash took place around 4:30 p.m. Thursday in Township 2 – Range 8, which is about 4 miles from the Lincoln exit. The truck, which was hauling fuel, was driven by Kenneth Gray, 59, of Bucksport.

McGovern also was the mother of an infant, who was elsewhere at the time of the incident, police said.