TOMAHAWK, Wis. (AP) -- An explosion in a paper mill storage tank killed three workers and injured another, leaving investigators trying to determine what caused the blast.
Packaging Corporation of America, based in Lake Forest, Ill., said the three who died were performing maintenance work atop a recycled fiber storage tank when it exploded about 1:30 p.m. Tuesday.
The employee who was injured but survived was standing on a platform at a lower level of the tank, the company said in a statement.
Company spokesman Ron Zimmerman said up to 10 people were working in the area when the explosion happened in what he described as "a storage tank for pulp." No fire resulted from the explosion, he said.
The cause of the explosion is under investigation, the company said.
Identities of the victims were not released. Zimmerman declined to give ages or other information about them.
Capt. Mike Drury, from 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.
"There was an explosion and a couple of employees were trapped for a short period of time," Drury said.
The mill is part of a large complex spread over several blocks, Drury said.
The main part of the mill remained in operation Tuesday afternoon.
Workers walked out with somber faces, refusing to comment.
One man was met by a woman in a car, and they embraced and hugged for nearly a minute. The man talked with her for several minutes and then went back into the administrative office, telling a reporter he couldn't comment.
Another worker said the explosion happened in a rear area of the property.
He said employees had been told not to discuss details of the explosion with reporters, referring questions to Zimmerman.
Carol Levanetz rushed to the plant from Rhinelander after hearing about the explosion.
Her son, Bryan Papineau, was OK, she said after checking at the mill's main office. She learned the explosion happened two hours before his work shift started.
"Now I just have to get my stomach settled," she said. "I am just so sad for those other people."
Packaging Corporation of America, which makes containerboard and corrugated packaging products, operates four paper mills and 67 corrugated product plans in 26 states. According to the company's Web site, it employs 8,350 people nationwide and posted sales of $2.3 billion last year.
The Tomahawk mill has three semi-chemical corrugating machines and produces more than 572 million tons annually, the company said.
AP writer Carrie Antlfinger in Milwaukee contributed to this report.