NASHVILLE (AP) - Officials at a Tennessee metal powders factory where five people died in accidents last year knew about the explosive danger of dust that coated much of the plant but did little to reduce the risk, a federal investigative agency concluded.

The U.S. Chemical Safety Board on Thursday released its final report on three accidents over the first five months of 2011 at a Hoeganaes Corp. plant in Gallatin, about 25 miles northeast of Nashville.

The report found that the company was first prompted to test the dust problem in 2008 when it was pointed out in an insurance audit. Tests in 2009 and 2010 found the dust was combustible, and the company developed an aggressive plan of safety improvements, the majority of which had planned completion dates before the 2011 flash fires.

But Chemical Safety Board investigator-in-charge Johnnie Banks said the plan "was not implemented in ways that would manage the dust."

Much of the information in the final report had already been made public, including the causes of the three accidents. Airborne iron powder ignited and caused flash fires in January and March. In the case of a May fire, hydrogen leaking from a corroded pipe exploded and lofted dust into the air that then created a fireball.

The Safety Board criticized plant management for relying on inadequate safety measures, such as flame-resistant clothing, to protect workers.

"The injured and fatally injured employees were wearing the Hoeganaes-designated FRC at the time of the 2011 flash fire incidents," the report concluded.

In a news release, Hoeganaes, based in Cinnaminson, N.J., said it has since taken action to prevent future accidents and ensure the plant operates at "world class standards." Those actions include "the development of an industry leading powder metal dust management system."

Banks said the plant did show noticeable improvement between the time he first v isited in early 2011 and his last visit in August, but he was not prepared to speculate on whether the plant was now completely safe for workers.

"I was afraid every time I went in there," he said, "and my team was too. You're walking through there and you're at a site where someone has suffered horrific injuries."

Currently there are no federal regulations specific to controlling the hazards from combustible dust in general industry, so any safety improvements companies chose to make are voluntary.

Federal figures show that deadly explosions from finely powdered food, wood, metals and chemical happen each year in the U.S., killing and maiming multiple workers.

In 2007, the Chemical Safety Board recommended that the U.S. Occupational Health and Safety Administration develop rules for preventing dust fires and explosions. Those rules still are under development.

That frustrates Safety Board Chairman Rafael Moure-Eraso. At the Thursday news confere nce, he said accidents like those at Hoeganaes are preventable, but many companies are not going to take the necessary safety measures unless they are required to.

In the case of Hoeganaes, the company knew the dangers and "decided to take the risk," he said.