JACKSONVILLE, Fla. -- Flying debris from a deadly December chemical plant explosion injured more than a dozen people other that those originally reported, federal officials said Friday.

The Dec. 19 explosion at T2 Laboratories Inc. killed four workers and injured 33 people, investigators from the U.S. Chemical Safety and Hazard Investigation Board said.

Initial reports said only 14 people were injured, but the board said Friday 19 others had minor cuts, bruises and hearing loss.

"Most of the injuries occurred offsite when a powerful blast wave swept through surrounding businesses," the board said.

Leon Bolden said the force of the explosion threw him to the ground.

"I injured by back, my neck, my arms -- it gave me an excruciating headache," Bolden said. "I thought I was dead."

Robert Hall, investigator-in-charge, said investigators discovered the additional injuries after interviewing people in the area surrounding the blast, which was comparable to a ton of TNT and blew debris as far as a mile away.

Hall said preliminary findings indicate the accident was caused by a runaway chemical reaction that occurred during the production of a gasoline additive. The explosion and resulting fireball caused a 2,000-foot mushroom cloud.

The CSB announced Friday it had finished its on-site investigation of the explosion. While they have not said what caused the deaths of four workers, they said that at the time of the blast, about 1,000 pounds of sodium were in an over-pressurized reactor. Hydrogen gas, a byproduct of the chemical reaction, was what created the fireball that was seen for miles around the North Jacksonville plant.

The CSB will conduct lab tests before releasing its final report. They said the goal of their work is not only to discover what went wrong, but to prevent it from happening again.