WORCESTER, Mass. (AP) — An 18-month old boy is dead after a family car parked at the top of a sloping driveway began to roll back as his mother was picking up the toddler from the backseat, knocking them both to the ground and running over the victims in Worcester.

Police say in a statement Saturday that Bennett Roaf died of his injuries Friday evening, hours after the crash occurred at the family's Chester Street home. An autopsy will determine the exact cause of death.

The toddler's 33-year-old mother was treated for a shoulder injury and has since been released from the University of Massachusetts Medical Center.

Police say initial investigations show that the open rear door knocked the mother and her son as the car began to roll back.

The investigation continues.