UPDATE AUG. 31 at 8:30 p.m.:
Investigators Friday night sought the cause of a fireworks explosion that badly burned one person and injured two others in The Diamond Stadium parking lot.
The fireworks went off unexpectedly about 5 p.m. in a dirt lot near The Diamond at Campbell Street; a crew was setting up a fireworks display for Storm Baseball's regular "Friday Night Fireworks" show.
"An explosion in the product occurred. We don't know what the cause was, but [it] was a catastrophic explosion that consumed the vast majority of the product," said Steven Beach, Cal Fire Battalion Chief.
Two employees from a fireworks display company and one inspector from Cal Fire were setting up the show and were injured in the accident, Beach said.
A fireworks company employee suffered first- and second-degree burns over 80 to 90 percent of his body, Beach said.
He was airlifted from the scene to a nearby hospital. Another employee and the firefighter suffered less severe burns and were both taken to area hospitals by ambulance.
The victims' names were not released.
As of 8 p.m. Friday, fire crews and the Riverside County Hazardous Device Team were on scene, disposing of remaining unexploded material.
Beach said crews were proceeding carefully because investigators don't know what triggered the explosions.
Storm Baseball General Manager Chris Jones issued a statement on the team's Facebook page Friday evening.
"We regret to announce that tonight's fireworks show has been canceled due to an unfortunate accident involving setup of the fireworks show.
"Our thoughts and prayers are with the fireworks crew and all those involved. [The] game will still go on as scheduled at 7:05 p.m."
The explosions triggered a small grass fire that burned less than a quarter acre and was quickly contained by fire crews.
According to a Patch reporter on scene, a black Toyota truck -- which sustained extensive damage in the blaze -- and a Ryder truck were being used to set up the fireworks for the game.
--Daniel Lane contributed to this report