Four workers suffered burns after a grain-dust explosion at an Arizona pet-food plant Sunday.

KSAZ reported that one of the workers was in critical condition as a result of the blast at the Nestle-Purina plant in Flagstaff. Two other workers were released from the hospital, while the condition of the fourth is unknown. Flagstaff Fire Dept. Captain Bill Morse said that the four men were welding on the fifth floor of the complex’s seven-story grain elevator when their torches sparked grain dust and set off the explosion at around 5 p.m. local time.