ATCHISON, Kan. (AP) — Federal investigators say ordinary key rings that were missing from chemical storage tanks were a primary reason a chemical cloud spread over Atchison in 2016, sending more than 140 people to the hospital.

MGPI Processing used metal key rings to secure pipelines to chemical storage tanks at its Atchison plant. The U.S. Chemical Safety and Hazard Investigation Board says in a report released Wednesday that in October 2016, an employee opened one line to take delivery of sulfuric acid. Two other lines were open because the rings were missing.

The Kansas City Star reports a delivery driver, expecting to find only one unlocked line, sent sulfuric acid into a tank containing sodium hypochlorite. The incompatible chemicals erupted into a cloud that spread over the Atchison area.