JACKSONVILLE, Fla. -- The explosion that killed four workers at a chemical plant on Dec. 19 was equal to a blast that would be created by a ton of TNT and blew debris as far as a mile away, a federal investigator said Thursday.

"The blast at T2 (Laboratories Inc.) was one of the most powerful ever examined by the Chemical Safety Board," said Robert Hall, investigator-in-charge from CSB, a federal agency that investigates major chemical accidents at industrial sites.

The blast occurred Dec. 19 as workers were mixing chemicals in a batch reactor to produce an additive to increase the octane rating of gasoline.

There were reports of loud hissing, vapor venting and then the explosion of the reactor vessel, which indicates excess temperature and pressure inside the reactor, Hall said.

After the rupture, the flammable contents mixed with air and ignited releasing large amounts of thermal energy, Hall said.

One of the pieces of debris, the top of the pressure vessel, which had walls three inches thick, and weighs several hundred pounds was found about a quarter-mile away, Hall said. Other parts were found as far as a mile away.

The cause of the explosion has not been determined and Hall said it was too soon to determine if it was caused by human error, mechanical error or a combination of both.

Investigators said it remains too dangerous for them to enter the debris field of the failed reactor and hazardous chemicals remain in the area.

So far, the investigation has been confined to surveying the nearby businesses, documenting blast damage, identifying injuries, interviewing injured workers and collecting security camera videos.

The CSB also plans to conduct reactive chemistry testing using T2's recipes to better understand what went wrong inside the reactor. A blast modeling team is also trying measure the exact nature and extent of the blast damage.

It may be several months to a year before the investigation is completed and the cause is determined, Hall said at a news conference.

Killed in the blast were Robert Scott Gallagher, 49, who co-owned T2 Laboratories Inc.; Charles Budds Bolchoz, 48; Karey Renard Henry, 35; and Parish Lamar Ashley, 36. All were from Jacksonville, the sheriff's office said.

One person remains hospitalized, although 14 people were injured by the blast, Hall said.

Hall said the company is cooperating in the investigation.

A person who answered the telephone at T2 Thursday and would not give his name said he could not comment since he did not know the contents of Hall's briefing.