MOUNT PLEASANT, Texas—Two people were killed Thursday morning when a tractor-trailer veered off Interstate 30 here, slamming into an overpass bridge pillar at an estimated 65 mph before erupting in flames, law enforcement authorities said.

The impact forced the truck’s engine through the cab and into the trailer.

The driver and a passenger were pronounced dead at the scene, said Texas Department of Public Safety Trooper Daniel Crooks.

The collision knocked the concrete off the pillar, exposing the rebar steel and causing the pillar to lean. Rebar steel is us...