JACKSONVILLE, Fla. -- Investigators charged with finding out what caused Wednesday's explosion and fire that killed four people told U.S. Sen. Bill Nelson that something went wrong while octane booster was being mixed at a Northside chemical plant.

Six federal agencies, led by the U.S. Chemical Safety Board, joined Jacksonville homicide detectives and the state Fire Marshal's Office sifting through what remains of T2 Laboratories, which exploded in a mushroom cloud.

Robert Scott Gallagher, co-owner of the facility that makes fuel additives and solvents, was among the dead. Also killed were Charles Budds Bolchoz, 48; Karey Renard Henry, 35; and Parrish Lamar Ashley, 36.

Fourteen other people were hospitalized for chemical exposure or their injuries after the blast.

"This octane booster ... is a fairly stable product, but to get to it, you use these highly energetic chemicals," Nelson said Friday morning. "A lot of heat is generated and a chemical process starts that can get in a runaway condition."

The fire was so intense that two of the bodies were not recovered until the next day.

"This particular accident most likely has the highest rank of any we've looked at in 2007 and one of the highest that the agency has investigated," said Bob Hall, who is heading a six-member CSB team. "This is a very serious accident, compared to what is normally seen in the industry."

Hall said the CSB's final report into what caused the blast and inferno would likely take six months.

"I just want to know what happened," Ashley's sister, Renea Stevens, said on Friday. "I never question God why -- because he has a reason for everything -- I just want to know what happened. Why wasn't he able to get out?"

Safety officers were inspecting earthquake-like damage to surrounding businesses to see if and when they will be allowed to reopen.

Up to 1,000 employees are unable to get to their jobs and Faye Road between Alta Drive and New Berlin Road remain closed to all but official traffic while the street is cleared of debris and the JEA replaces power lines damaged.

At 4:45 p.m. Friday, police officially declared the explosion to be a catastrophic industrial accident and turned the site over to the the CSB, OSHA and the Moran Environmental Group.