According to the Federal Aviation Administration, a single-engine Piper PA-32 crashed around 11:40 a.m. during the NHRA Drag Racing finals.
The crash prompted the NHRA to halt the competition, but the event started up again around 1 p.m.
The NHRA said the plane was apparently attempting to land at Brackett Field Airport, which is located just west of the track.
"The NHRA has been working closely with the Pomona Police Department and LA County Fire Department and has been guided to continue racing," read a statement from the group.
The Los Angeles County Fire Department said there were four people on board the aircraft, all of whom were injured. Two of them are in critical condition while the remaining two suffered moderate injuries.
The fire department said the pilot missed the dragstrip itself and the grandstand area and no spectators were injured.
The exact cause of the crash remains under investigation.