South Africa’s trade union Solidarity said on Thursday four workers died at the Newcastle plant of the local unit of the world’s top steel maker ArcelorMittal.

Solidarity spokesperson Reint Dykema said one of the workers broke his neck after falling from a scaffolding while preliminary investigations showed the other three workers could have been “overpowered by oxygen”.

“The workers were carrying out maintenance (works) when the accident happened and investigations are continuing to find out the exact cause of their deaths,” Dykema said. He said another person had been admitted to a local hospital in a critical condition.