Four people were killed in two traffic crashes within hours of each other on Interstate 15 near the California border early Sunday, authorities said.

The first wreck occurred about 3 a.m. and involved a wrong-way driver.

A Ford Explorer was northbound in the southbound lane of the interstate when it collided with a Dodge Dakota pickup about three miles from the state line, said Kevin Honea, spokesman for the Nevada Highway Patrol.

The Dakota burst into flames, and a woman and a boy -- 11 or 12 years old -- were inside the truck and were killed, Honea said. Honea said he didn't know whether the boy was the woman's son.

The man driving the Explorer was pulled from the SUV by a Nevada Highway Patrol trooper and a member of an ambulance crew before the SUV also caught fire, Honea said.

But the driver of the Explorer also was pronounced dead at the scene, he said.

There was no initial evidence indicating that the SUV driver was suicidal or that he was intoxicated, Honea said Sunday afternoon, but the investigation is continuing.

About 5:20 a.m., as traffic was backed up on the southbound side of I-15, a blue Jeep SUV slammed into the rear of a stopped tractor-trailer, about two miles north of the first wreck, Honea said.

The driver of the Jeep, a man, was killed, Honea said.

"The guy never stopped," Honea said, explaining that the Jeep left no skid marks so the driver apparently didn't brake before hitting the 18-wheeler.

It was unclear whether the Jeep's driver was distracted or had fallen asleep, Honea said.

I-15 was backed up in both directions near the state line until about 12:30 p.m. Sunday , Honea said.