Bonita Springs, Fla. ? A tractor-trailer filled with fireworks for the town’s Fourth of July celebration exploded as workers unloaded the truck Wednesday, unleashing several thunderous blasts that shot colorful flames into the sky. At least four people were killed and two were injured.

“It just all happened at once,” said Kevin McKenzie, who was mowing grass near the truck when it blew up. “Immediately it was all the fireworks going off with all the colors and the flames.”

Bonita Springs City Manager Gary Price said the blast happened at 2:10 p.m. as workers were transferring part of the load of fireworks to another truck.

The blast occurred on a tip of vacant land at a state park in Bonita Springs, a city of about 30,000 near the Gulf of Mexico.

Hours after the blast, smoke rose from a blackened pine tree and the charred remains of the two trucks, which had been parked back-to-back.

The blast left two people injured, including a man in critical condition who was transferred from the trauma unit of Lee Memorial Hospital to a Tampa hospital, said Alex Reichart, an administrative supervisor with Lee Memorial Health Systems. A woman was listed in good condition at Memorial’s HealthPark campus, he said.

The bodies remained at the scene overnight because authorities were concerned about the safety of the area and did not know whether all the fireworks had detonated, said Lee County Sheriff’s spokesman Sgt. Michael Maschmeier.

“Right now it’s still a dangerous situation,” Maschmeier said.

The state Fire Marshal’s Office and the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives were handling the investigation.

The fireworks were from Sunset Fireworks of Dittmer, Mo.

Price said the city spent $20,000 for a planned 30-minute show this year.

“It’s just hard to believe,” Price said. “The event you’re setting up for everybody’s enjoyment to celebrate our anniversary of our country results in such tragedies. It’s just hard to believe.”