A 4-year-old boy buckled into a car safety seat suffered life-threatening head injuries Monday after his grandfather suddenly veered off a road and struck a tree in Bethpage, Nassau police said.

Their 2008 Nissan Altima was going north on Stewart Avenue by Arthur Avenue when the driver, 65, lost control of the car and careened over the sidewalk about 1:20 p.m., police said.

He and his grandson, who was buckled in a child seat in the back, were taken to hospitals, but the grandfather’s injuries were not life-threatening, police said. The names of the two were not released.

The driver did not immediately remember what had happened, police said.

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Homicide detectives did not find any apparent criminality, and the results of field and blood tests did not show any sign of alcohol or drug impairment, police said.

The car was taken for brake and safety checks, police said, and the cause of the crash remains under investigation.