By The CNN Wirestaff

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Reno, Nevada (CNN) -- "Mass casualties" were reported at an air show after a plane crashed Friday into the box seat area in front of a grandstand at the National Championship Air Races and Air Show in Reno, Nevada, a spokesman for the show told CNN.

Mike Draper said he's been told there are "likely fatalities," but it has not been determined how many or who they may be.

The pilot, identified as Jimmy Leeward, a real estate developer from Ocala, Florida, was likely killed in the crash, the show said in a statement. He was flying a P-51 Mustang. The event has been canceled and a memorial service will be held for the pilot Saturday afternoon, the show said.

A Reno, Nevada, hospital said Friday evening on its website that it had received a "total of 21 patients" after the plane crashed. "No further patients are expected at this time," the Renown Regional Medical Center reported at 5:45 p.m. PT. The conditions of those patients were not released.

Jimmy Leeward, pilot of the P-51 "Galloping Ghost" from Ocala, Florida, likely died in the crash.

Video from the scene, posted on YouTube, showed a plane plummeting from the sky, sending up clouds of dust and debris.

Shocked spectators rose to their feet.

Fred Scholz, a witness, said he saw the plane seemingly "coming straight down."

"I thought he was coming right on top of us," he said, noting that the aircraft ultimately hit about 100 feet from him, in an area of box seats, filled with folding chairs.

"Luckily there was no fire, which surprised me," said Scholz, a 59-year-old from Truckee, California, who says he goes to air races ever year. "It just happened so quickly."

Another witness, Greg Mills, added that the pilot "didn't have enough altitude to pull up," with the aircraft shuddering before slamming to the ground about 50 to 75 yards from where he was standing.

"We got showered by little stuff," said Mills, who works at the Pacific Aviation Museum based in Pearl Harbor, Hawaii, describing a chaotic scene afterward. "Everybody was running."

The plane, called the "Galloping Ghost,"(Above) was taking part in a qualifying round in the "unlimited class" division of the air race when it went down around 4:15 p.m. PT Friday, Draper said. The final rounds had been slated for the weekend.

The aircraft was about a lap into the race when the pilot sent out a "Mayday signal" and pulled "out of the lap" before crashing into a box seat area that is in front of a grandstand at the "very large race field," located about 15 miles north of Reno, Draper said.

"It was clear that (the plane) was pulling up and was in distress," he said.

He added that authorities do not yet know why the aircraft went down.

Besides the Federal Aviation Administration personnel already there to assist the National Transportation Safety Board in any investigation, the show's spokesman said that National Guard members -- who were on-site practicing before the incident -- are helping emergency personnel to clear the area.

"There are people still being escorted out of that area with various injuries," Draper said.

Local fire officials are reporting multiple fatalities and critical injuries, according to Ian Gregor, a spokesman with the FAA.

He said the aircraft did not catch fire, but there was a wide debris field following the crash.

Gary Arthur, another witness to the accident, said the plane wavered a few times, suggesting the pilot had tried to get control before going down.

"Just the sound of the impact was unbelievable," he said. "Almost can't believe you saw what actually happened."

In 2007, a mid-air collision during the annual National Championship Air Races in Reno claimed the life of one of the two pilots involved. The collision was the latest in a string of accidents that plagued the races that week, killing three pilots.

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CNN's Mike Ahlers, Kelly Marshall Smoot, Greg Morrison and Nick Valencia contributed to this report.