Four workers were critically injured by an explosion at Dow Chemical’s plant in North Andover, Massachusetts, USA, on Jan. 7. A fifth person was treated at the scene but was not seriously injured.

The blast occurred in a lab and involved the chemical trimethylaluminum (TMA), which is said to have reacted with water. A colorless liquid, TMA is reactive and a fire and explosion hazard. It is a key component in making LED lights and electronics, such as circuit boards.

The local fire marshal said this was one of the more complex and larger hazmat incidents it had responded to in recent times.

As of Friday Jan. 8, federal and state investigators were continuing their investigations, and two inspectors from the Occupational and Safety Health Administration (OSHA) had been dispatched to the scene.

A Dow Chemical spokesperson told local media there were no risks to the surrounding community.

This was the second major accident at the company’s Andover plant in two years. An employee working with the substance trimethylindium, also known as TMI, was killed in another lab explosion in 2013. The chemical was found to have been exposed to air or a cleaning solution.

OSHA completed its investigation into that incident just three months ago, finding that Dow Chemical had failed to ensure the equipment it used was adequate to prevent explosions. The company was fined $17,000 for the safety violations.