California Highway Patrol officers said Monday they were investigating why a 29-year-old motorist from the San Diego area crossed the U.S.-Mexico border only to turn around at a checkpoint and lead authorities on a deadly, 100 mph pursuit.

The 38-mile, wrong-way run ended Sunday on busy Interstate 5 with a head-on crash that killed the man and a couple.

Drugs, alcohol, weapons or money do not appear to have caused the man to turn and run from authorities, CHP Sgt. Curtis Martin said.

The names of those killed were not immediately released.

Martin said the couple had a rental car and luggage and appeared to be beginning or ending a trip. A number of other people on the freeway managed to swerve and avoid the wrong way driver, he said.

Customs and Border Protection officers had tried to catch the man, who was driving a 2006 Mercedes.

"He drove northbound in the southbound lanes while the Customs and Border Protection officers went northbound in the correct lanes," Martin said.

CHP officers were trying to stop the car or put down spike strips. But the man was going so fast it was hard to get into position.

During the crash, the couple's car flipped over, landed on its roof and burst into flames. Leaking gasoline created a line of fire across all lanes of the roadway and some of it went into a storm drain, Martin said. A hazardous materials team was called to clean up.

The Mercedes driver died instantly. His car did not overturn or catch fire, Martin said.

The freeway was shut down for 6½ hours during the investigation and cleanup.