An 86-year-old man and his 19-year-old granddaughter died Thursday afternoon when, for unknown reasons, their sedan crossed into oncoming traffic on La. 67 and slammed head-on into an 18-wheeler.

Both vehicles flew off the road and were engulfed in flames, said Trooper First Class Jared Sandifer, a state police spokesman.

William Blalock and Ariel Foster, both of Liberty, Mississippi, were heading north on the highway near the Mississippi state line about 12:30 p.m. when Blalock veered into oncoming traffic and struck Sidney Rollins III, 46, also of Liberty, who was driving a tractor-trailer, Sandifer said.

Rollins received only minor injuries. No driver impairment is suspected, Sandifer said. Blalock and Foster were wearing seat belts.