CLOVIS, N.M. (AP) — Investigators are trying to determine what caused a mechanical failure that resulted in 20 workers inhaling toxic fumes at a cheese manufacturing plant in eastern New Mexico.

The Curry County Sheriff’s Office reported that more than a dozen workers were transported to area hospitals by ambulances and private vehicles Monday morning after acid mixed with chlorine to create the fumes. Two of the workers were listed in critical condition.

Crews responded after a strong smell was reported at the Southwest Cheese plant. A hazardous materials crew with the Clovis Fire Department helped to clear the scene and production resumed later that morning.

Southwest Cheese said in a statement issued Monday that early investigation indicated the cause of the incident was a chemical overflow that occurred due to a mechanical failure in one room at the plant and that the vapors affected employees who were working close by.

The company said the affected area was quickly closed off per its incident response plan.

Southwest Cheese has been operating since 2005. It processes billions of pounds of milk every year, producing more than 588 million pounds (266,712 metric tons) of block cheese and more than 36 million pounds (16,329 metric tons) of whey protein powders.