GALLATIN, Tenn. (AP) — Three workers have been critically burned in an explosion at a Middle Tennessee chemical plant, and federal investigators are on their way to the scene.

The Friday morning accident that injured five is the third this year at the Hoeganaes (HAY'-gan-eez) Corp. plant where a January flash fire killed two workers, one of whom died last week. The Gallatin facility makes metal powders, and the two previous accidents occurred after flammable dust in the air combusted.

The plant already was under investigation by the U.S. Chemical Safety Board. An investigator criticized the company two weeks ago for not adequately addressing the danger posed by the dust.

On Friday, a spokeswoman said CSB officials were on their way.

A message left at Hoeganaes headquarters in Cinnaminson, N.J., was not immediately returned.