An explosion heard miles away sent a ball of fire over the historic Atlas Foundry Saturday afternoon, shutting down a major highway and cutting power to the city's industrial area.

Four people were taken to a hospital - the driver of a propane truck and three employees of the foundry - after two propane tanks exploded around 3 p.m., said The News Tribune newspaper.

The driver of the propane truck was airlifted to Harborview Medical Center in Seattle, where spokeswoman Susan Gregg-Hanson said he was in critical condition Saturday evening. The three other injured men were listed in stable condition at St. Joseph Medical Center in Tacoma.