NASHVILLE (AP) - An investigation into a May explosion and fire that killed three workers at a Gallatin metal powders factory found that a hydrogen gas leak came from a pipe that was not normally meant to be pressurized.

The Tennessee Occupational Safety and Health Administration issued an $80,000 fine to the Hoeganaes Corp. for 23 serious violations. The plant was fined $49,200 just a week before the May 27 accident for violations related to flash fires in January and March that left two workers dead and a third injured.

Officials at the company's headquarters in Cinnaminson, N.J., did not immediately return a call requesting comment. Last week, Hoeganaes released a statement saying it was working to improve safety "to ensure that similar accidents will not happen again."

According to the TOSHA report released Monday, workers heard a gas leak and called maintenance to investigate. The gas pipes were located in a trough under metal fl oor plates. To get to the leak, workers attached chains to one of the plates and raised it with a forklift.

One of the workers told investigators that "as the cover broke loose, he witnessed sparks generated along its periphery and immediately there was an explosion and fire."

Two employees who had just entered the building reported running back out upon seeing a fireball coming toward them. They then saw "the fireball go past the doorway and then they saw a dust cloud in the building that obscured their vision."

A federal investigation by the U.S. Chemical Safety Board has found that the hydrogen explosion knocked lose iron dust that had accumulated throughout the building. That dust then ignited, creating a flash fire.

The Safety Board also blamed the two accidents earlier this year on the accumulation of combustible metal dust.

TOSHA investigators probing the May accident found the trough below the floor contained pipes supplying hydrogen and nitrogen as well as a vent pipe that was routed through the roof and into the atmosphere.

"The vent pipe was not intended to carry any pressure, except during the purging of the furnaces," the report states. But that is the pipe that was leaking hydrogen. The report found "the most likely reason that the pipe was pressurized was that venting valves were mistakenly opened to purge a furnace and never closed." The vent pipe also was "in a severely corroded condition."

The report faults the company for failing to evaluate and plan for the dangers posed by the hydrogen supply and not inspecting or maintaining the gas pipes, which were located where they were exposed to water. It also faults Hoeganaes for failing to follow its own emergency action plan after the explosion. The report instead describes a chaotic scene in which no one was clearly in charge and employees took it upon themselves to fight the fire and rescue colleagues.

While the report found that "co mbustible dust did play a role in the fire spreading to other areas of the plant, including a maintenance cart that was parked about 20 feet from the hydrogen fire" and contributed to the burns of some of the employees, TOSHA did not cite the company for violations related to the dust. That's because several of the violations from the January and March accidents refer to the dust accumulation and that citation is still open while Hoeganaes contests it.

The company has until Dec. 23 to correct the violations.

In a statement last week, the company said it is developing what it called "an industry leading powder metal dust management system" at the recommendation of outside experts who undertook a comprehensive safety review of the plant.

Hoeganaes also says it is upgrading the electrical systems, replacing the gas and air supply system and upgrading "gas management and hydrogen detection systems."