INDIANAPOLIS (AP) — The death of a 13-year-old motorcycle rider at Indianapolis Motor Speedway cast a shadow over Sunday’s races at the historic track and prompted mourning competitors to defend the development system for the dangerous circuit.

Peter Lenz of Vancouver, Wash., fell off his bike during the warmup lap for the day’s first race at Indianapolis and was run over by another motorcycle, driven by a 12-year-old. Medical workers immediately placed Lenz in a neck brace, put him on a stretcher and began chest compressions while taking him to a hospital.

Several hours later, he was pronounced dead.

The Marion County coroner’s office said Lenz died from blunt force trauma. More details could be released today following an autopsy on the youngest driver or rider ever killed at the 101-year-old speedway.

‘‘Peter passed away early this morning when he was apparently struck by another rider,’’ read a posting on Lenz’s Facebook page, which was signed ‘‘Dad.’’

‘‘He passed doing what he loved and had his go fast face on as he pulled onto the track,’’ the posting said. ‘‘The world lost one of its brightest lights today. God Bless Peter and the other rider involved. 45 is on another road we can only hope to reach. Miss you kiddo.’’

Lenz rode the No. 45 bike, and his father was at the track Sunday.

It was the first death at the track since IndyCar driver Tony Renna was killed in testing in October 2003.

Lenz had emerged as one of America’s youngest rising stars.

At age 11, he earned the ‘‘expert’’ license from the American Federation of Motorcyclists, and in March 2009, Lenz became the youngest rider ever to win an AFM race. This year, competing in the U.S. Grand Prix Racers Union series, Lenz had four wins, five podium finishes and was leading the MD250H classification in points.

The grown-up resume just didn’t match his appearance. Listed at 4-foot-11 and 81 pounds, Lenz’s face was clean-shaven and smooth, and he described his profession as ‘‘kid.’’

‘‘Our hearts go out to the parents, family and friends of Peter Lenz,’’ speedway CEO Jeff Belskus said in a statement. ‘‘Words cannot adequately express the sadness of our company and our employees about this tragic incident, and Peter is in our thoughts and prayers.’’

It wasn’t the first time Lenz was involved in a serious crash.

A mechanical failure last season left Lenz with four broken bones and a severed radial nerve — all of which required surgery. By November, Lenz was healthy again and by spring, he was back in Victory Lane.

The fatal accident almost certainly will spark a debate about how young is too young for racers to be competing on one of the world’s best-known tracks, whether it’s inside a car or riding a motorcycle capable of exceeding 120 mph in a straightaway.

The USGPRU sanctions races across the nation, billing itself as a development circuit for motorcycle racers from ages 12 to 18. The hope is these riders eventually will compete in a world-class series.

Lenz seemed to be on his way to the top before the accident. He was struck by 12-year-old Xavier Zayat, of Flushing, N.Y., who escaped injury.

‘‘We are deeply saddened by this tragic loss, but know that Peter is racing even faster in the sky,’’ the Lenz family said in a statement. ‘‘Our thoughts and prayers are now with the other racer and his family, who were also involved in this tragedy.’’

Racers insist age has never been the issue.

American Colin Edwards was running 250cc bikes at age 17, and Indy MotoGP runner-up Ben Spies was competing on the 125cc circuit at age 12.

‘‘That’s not like a bike too big for him, you know, I mean this is our sport, we chose to do it,’’ said American Nicky Hayden, the 2006 world champ who called the death ‘‘terrible.’’

‘‘I mean, sure, we know going in the consequences.’’

But are new rules needed?

Those in racing circles say no, comparing the sport to other potentially dangerous sports such as football and gymnastics. When asked about running at Indy, speedway officials pointed out that the series has stops at potentially more dangerous tracks.

USGPRU officials said this was the first fatality in the series in nine years.

Still, racing conditions were not ideal this weekend at Indy.

Hot, dry weather turned the bumpy, 2.621-mile course into a slick track that tested the world’s best riders.

Reigning world champ Valentino Rossi fell four times, including a spill Sunday morning during a 20-minute warmup session. Sunday’s Moto2 race was shortened after a big wreck on the first lap took out four drivers.

‘‘This was the most difficult race for everyone,’’ MotoGP winner Dani Pedrosa said. ‘‘The conditions were very hard because of the heat and the asphalt was very greasy.’’

After a brief delay to clear the track, the USGPRU race was restarted. Had they known the severity of the injuries, one official said they would have considered canceling the race. The three other races all started on time, though Lenz was never far from the racers’ thoughts.

‘‘This is an ugly, terrible part of this sport,’’ USGPRU chief steward Stewart Aitken-Cade said. ‘‘You do what you can to stop it from happening as best you can. That’s really all that you can do.’’

‘‘Any time a racer is injured in this way and loses his life, it’s tough, adult or child,’’ Aitken-Cade added. ‘‘It just makes it especially difficult when it’s a young guy like Peter.’’