A storage tank explosion at a northern Wisconsin paper mill Tuesday killed three workers and injured a fourth, the company that owns the mill said. Packaging Corp. of America said in a news release that the three workers were fatally injured when a tank used to store recycled fiber exploded while they were performing maintenance on top of it. The injured worker was standing on a platform at a lower level of the tank when it exploded, the company said. The worker's condition was not immediately released. The cause of the explosion was not known and is under investigation, the company said. Capt. Mike Drury of the nearby Merrill Fire Department said his rescue crew was called to help free the employees about 1:40 p.m. but was called off before it arrived. Emergency workers on the scene used a crane to rescue them, he said.