GIBSONIA, Pa. (AP) - A wood chipper being hauled by a dump truck broke free and bounded across a highway, striking a minivan and killing a father and two of his young triplets, police said.

The dump truck was traveling north Thursday evening when the wood chipper crossed the center line of Route 8, a busy highway that snakes through Pittsburgh's northern suburbs.

The southbound minivan was carrying Spencer Morrison, 37, and his 4-year-old triplets, police said.

All three children were in safety seats, but one child, Ethan Morrison, was thrown from the vehicle in the impact, which tore a door from the van, police said.

Ethan was flown to Children's Hospital in Pittsburgh, where he was in critical condition Friday morning. His siblings, Garret and Alaina, were killed. Police were investigating how the chipper, which they estimate weighed about 5,000 pounds, came free. The road was very smooth, Northern Regional Police Chief Bob Amman said Friday