The 8:20 a.m. blast at the Williams Northwest Pipeline plant in the Washington town of Plymouth, along the Columbia River, also punctured one of the facility's two giant storage tanks for liquefied natural gas.
Benton County Sheriff Steven Keane said some gas leaked from the tank to the ground in a moat-like containment area and evaporated into the air, blowing away to the northeast. But it was a relatively small amount, he said.
"I think if one of those huge tanks had exploded, it might have been a different story," Keane said.
One of the five injured workers was transported to a Portland, Ore., hospital specializing in burns, he said. The other four were taken to Good Shepherd Medical Center in Hermiston, Ore., where spokesman Mark Ettesvold said they were treated in the emergency room for injuries that did not appear to be life-threatening.
Across the Columbia River, the blast shook the home of Cindi Stefani.
"It was just a very loud boom," she said. "I looked across the river and saw a giant mushroom cloud and flames at least a couple hundred feet high."
Animals on neighbouring farms were running around, she added.
"At that point we were pretty scared. I was thinking, 'We need to get out of here.'"
Deputies went door to door to homes and farms within a 2-mile radius, evacuating about 400 residents as a precaution.
Buses were provided for those without cars, and a shelter was set up across the river in Oregon at the Umatilla County Fairgrounds. As part of the evacuation, Highway 14 and railroad tracks were shut down.
Deputy Joe Lusignan said the voluntary evacuation could last overnight. No one was being prevented from returning to the evacuation area, which was calculated based on the damage expected if one of the two storage tanks blew up.
The one that that punctured has a capacity of 1.2 billion cubic feet, but it was not full, the sheriff said.
The fire at the plant was extinguished within a couple of hours. Williams spokeswoman Michele Swaner in Salt Lake City said all 17 or 18 company employees were evacuated and accounted for.
She added it was too early to determine the extent of the damage or the cause of the explosion.
Video taken by a Washington State Patrol bomb squad robot was being evaluated, and plans were being developed to send up a helicopter for an aerial assessment of the facility, authorities said.
A pipeline engineer with the Washington Utilities and Transportation Commission will investigate the cause of the explosion and communicate with the western region of the U.S. Department of Transportation Pipeline and Hazardous Materials Safety Administration, the commission said.
The state Pipeline Safety Program regulates 28 pipeline companies and inspects more than 24,000 miles of natural gas and hazardous-liquid pipelines in Washington.
Williams operates about 15,000 miles of interstate natural gas pipelines, according to its website. The Plymouth facility cools natural gas into a liquefied state, company spokesman Tom Droege said.
The liquefied natural gas facility is owned by Williams Partners' subsidiary Northwest Pipeline LLC.
There was no pipeline rupture, and no customers were affected, company officials said.
A secretary with the Patterson School District, about 7 miles away, said it provided three school buses to help with the evacuation. Students are on spring break, secretary Rachelle Munn said.