Three workers are in critical condition in the Vanderbilt University Medical Center burn center after an explosion Friday at the Hoeganaes Corporation’s manufacturing plant in Gallatin, Tennessee.

It was the third serious accident at the facility this year. A January 31 flash fire fuled by combustible iron dust killed two workers – including one who died from burn injuries earlier this month. A similar fire on March 29 injured one worker.

A total of five workers were injured in the latest blast, which is under investigation by the U.S. Chemical Safety Board (CSB). Inspections conducted previously by the CSB found “significant quantities of accumulated metal dust on surfaces within close proximity to the incident locations and elsewhere throughout manufacturing areas, including elevated surfaces,” according to a CSB statement. Additionally, laboratory tests showed that metal dust collected from various locations around the facility was combustible and capable of exploding when dispersed in air and confined.

The cause of the most recent accident has yet to be determined. The CSB team will be interviewing key witnesses, examining the incident scene, and requesting relevant documents.

The Hoeganaes plant produces iron powder for industrial customers and employs approximately 180 people.