TOMAHAWK, Wis. (AP) — 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 is under investigation, the company said.