Four people were injured when a small plane apparently trying to land at Brackett Field Airport in La Verne crashed near the track at the Pomona Dragstrip in Pomona on Sunday morning as thousands of spectators watched hot rod races.

The crash occurred about 11:10 a.m. in the 2700 block of Fairplex Drive, adjacent to the drag strip, the Los Angeles County Fire Department said.

It’s unclear what caused the single-engine Piper PA-32 to crash.

The Federal Aviation Administration said it likely will have a preliminary report today.

The airplane struck several vehicles on the ground, including a large recreational vehicle, but no bystanders were hit or injured.

The pilot and three occupants of the airplane suffered moderate to critical non-life-threatening injuries and were being treated at a nearby hospital, according to county Fire Department spokesperson Jonathan Torres.

No fire was seen, but fuel spilled all over the area. The plane crash occurred during the first round of Top Fuel eliminations. The races were delayed because of the crash, and the cars were removed from the drag strip.

An estimated 10,000 people were attending the National Hot Rod Association championship drag racing finals, which were placed on hold. Racing later resumed.

Norman Wade of Corona parked across the street before the crash. He wanted to watch the drag races from a nearby parking lot to avoid paying expensive ticket fees, he said.

Wade was in his car with the door open watching a plane flyover after the national anthem played when he noticed two smaller planes heading for a landing at the Brackett Field Airport. Both planes appeared like they would land safely, but then one stalled and its nose dropped before it crashed into the Fairplex parking lot, Wade said.

“He just dropped right out of the sky,” Wade said.

The crash caused a loud bang, a plume of dust and debris and some smoke, he said.

Wade feared a fire would erupt, but after the dust settled nothing else happened, he said.

Greg Correa, who arrived early Sunday to see the races, said watching the plane go down was like witnessing a car crash.

“It was like slow motion just coming in,” he said. “There was no explosion or nothing.”

Kathy Mickelson, who saw the crash from across the street, said the spectators were shocked.

After the crash, firefighters from La Verne who had been standing on their trucks to watch the race from the parking lot sprung into action, Mickelson said. They jumped in their trucks, turned their lights on and rushed to the scene.

Investigators from the National Transportation Safety Board were on their way to conduct an investigation into the crash Sunday afternoon.

Staff writer Tim Haddock contributed to this report.