Leffler died after the crash during the first heat at southern New Jersey's Bridgeport Speedway, a dirt track about 15 miles southwest of Philadelphia.
A spectator, Chris Taitt, said he was at the race but looking the other way when the crash occurred. He says Leffler had been in second place, apart from other cars when his car slammed into a wall at the track's fourth turn.
Taitt, 40, of West Deptford, said the wing on the car was "flattened like a pancake," and the seat appeared to be displaced.
Leffler, 37, a native of Long Beach, Calif., won twice on NASCAR's Nationwide Series and finished in both the Indianapolis 500 and the Daytona 500, two of car racing's signature events.
But apart from a last-place finish in Sunday's NASCAR race at Pocono, he had spent most of 2013 racing on dirt tracks.
The 410 Sprint Car race Wednesday promised $7,000 to the winner.
While it's in the minor leagues of car racing, it was the highest profile event so far this year, with better-known drivers, more expensive tickets and a bigger prize than the usual Friday and Saturday night events at the raceway.
Sprint car races can be more dangerous for drivers and spectators because the safety measures taken by series aren't at the same level. Many facilities lack the SAFER barriers that are standard in NASCAR and IndyCar, and the cars aren't always adequately protected.
Three drivers were killed last month in crashes on dirt tracks. Driver Josh Burton died of injuries sustained in a crash at Bloomington Speedway in Indiana; and two drivers were killed in a race in Nevada.
In March in California, two people were killed when a car careened off a dirt track and crashed on pit road.
While investigators tried to piece together what happened, the car racing world was remembering Leffler, recalling him as a loving father to his 5-year-old son, Charlie Dean, an open person and a versatile driver.
"Jason Leffler was a great racer and an even better friend," said Tony Stewart, a three-time NASCAR Sprint Cup Series champion who was once a teammate of Leffler's at Joe Gibbs Racing. "To not have him around to talk about whatever race one of us had just run, or were going to run, will be hard. My thoughts and prayers go out to his family, especially his son, Charlie, who Jason loved more than anything."
Leffler's representative, Spire Sports, said funeral arrangements are being made.
"Despite his many accomplishments, Jason still followed in the same footsteps of his heroes that would race anything, anytime. All Jason wanted to do was race. He was the life of every party and a true racer," Spire said in a statement. "We will miss Jason dearly and know that his family appreciates all the thoughts and prayers."
Panther Racing Owner John Barnes, for whom Leffler raced in 2004 and 2005, said Leffler had a "fierce competitive spirit and a devilish attitude. Jason was a small man with a huge right foot."