Earnhardt, who died instantly Sunday, probably was thrust into the steering wheel because he wasn't fully supported, said Dr. Steve Bohannon, head of emergency medical services at the Daytona track.

The belt the seven-time Winston Cup champion was wearing as part of his harness came apart near a buckle.

"We don't know how, when or where, yet," NASCAR president Mike Helton said at a news conference. "We will continue our investigation."

The company that made Earnhardt's harness, Simpson Performance Products Inc. in Mooresville, insisted that it has never had a problem with its seat belts. Company founder Bill Simpson, a former Indy car driver, implied that the belt that broke might have been installed incorrectly.

"Having tested and produced seat belts for the motorsports industry for more than 43 years, we have never seen a seat belt come apart in the manner that occurred," Simpson said in a statement. "Our seat belts, when properly installed, won't fail."

Richard Childress, Earnhardt's longtime car owner, didn't immediately respond to Simpson's statement.

Winston Cup director Gary Nelson held up a similar lap belt and described how the webbing that attaches the belt onto the car frame came apart.

He wouldn't say how the material came apart or whether it was cut, frayed or damaged in any other way.

"All we know conclusively is the belt came apart," Nelson said. "We've never seen it. We've talked to people in the business, and they say they've never seen it in 52 years of NASCAR racing."

Childress said earlier that the seat belts Earnhardt used at Daytona were standard and were new when the car was built last November.

The death of the 49-year-old racing great stunned the racing world and led to calls for better safety measures.

Another factor in Earnhardt's death could have been his old-fashioned open-face helmet. A closed-faced helmet might have made a difference, Bohannon said.

"It would have played a role," he said. "He would have had a different pattern of injuries."

Like most drivers at the race Sunday, Earnhardt also shunned the use of the U-shaped HANS device -- for Head And Neck Support -- which many drivers find bulky and uncomfortable. It fits around the neck and is attached by a strap to the helmet and frame of the car.

"I do support further neck and head restraints, but I'm not convinced the HANS device would have made a difference," Bohannon said.

Following the news conference, Dale Earnhardt Jr. made his first extended public comments since the race, in which he finished second moments after his father crashed. He will be on the track Saturday when practice begins for Sunday's race at North Carolina Speedway. "I miss my father and I cried for him out of my own selfish pity," he said. "We just have to remember he's in a better place that we all want to be." Earnhardt's widow, Teresa, expressed her gratitude to fans in an open letter published Friday by USA Today. "Remember the things about him that made you happy that you were his fan," she wrote. "Remember the man who loved life. He was the happiest person I know, and that can comfort us all."

The Chevrolet Kevin Harvick will drive as a replacement for the late Dale Earnhardt will sport the No. 29, along with a white paint scheme.

NASCAR doesn't retire numbers, but Childress said he would never again race Earnhardt's black No. 3 car.

Earnhardt was buried in a private service on Wednesday in his hometown of Kannapolis. On Thursday, thousands of people gathered to honor him at a memorial service.

Helton said NASCAR was not contemplating any safety changes for Sunday's race at North Carolina Speedway. He said experts were still looking at Earnhardt's battered Chevrolet and will also study the broken belt.

He said information on the broken belt will be passed on to crew chiefs in the Winston Cup and Busch series here and at a truck race next week at Homestead, Fla.

Note: The five-point system, which includes the shoulder harnesses, all connect to the lap belt. All of them are anchored at separate points on the race car, but the lap belt has an aircraft quality latch on it. They're five-inch wide belts and they're all interconnected. If the left lap belt broke, the shoulder harness and everything else would totally disconnect because it is also the latch -- or connecting side -- of the belt.