A propane tank exploded about 11 a.m. at the plant along Picnic Point Road between Mukilteo and Edmonds.
Firefighters arrived to find flames leaping from a truck and three buildings, said Leslie Hynes, a spokeswoman with Snohomish County Fire District 1.
"It was very instantaneous, the explosion," she said.
The injured people were rushed to Harborview Medical Center in Seattle. Doctors were treating five men, between the ages of 31 and 48, for burns mainly to their upper bodies, hospital spokeswoman Susan Gregg-Hanson said.
Four of the patients were reported in satisfactory condition a few hours later. One man was in serious condition and was expected to be moved to the intensive care unit. None of their injuries were considered life-threatening, Gregg-Hanson said. One man had been released by the evening.
The blast happened as a crane was moving a 1,000-gallon propane tank onto the bed of a truck, Hynes said. The weight of the tank shifted, and the movement apparently caused a valve to shear off the tank, which was filled with about 300 gallons of propane.
The truck driver attempted to warn everyone in the area about the leak before the tank exploded, Hynes said.
The blast sparked a fire that destroyed the truck and spread to nearby trees and buildings. Those fires were quickly extinguished. The truck's doors were nearly ripped off, and the bed was partially melted. Damage was estimated at $1 million, Hynes said.
The injured men were part of a construction crew working on the project, which is scheduled to be completed in November, said Mike Pivec, a manager at the Alderwood Water and Wastewater District. Construction work began in 2008 to double the sewage-treatment plant's capacity.
All wastewater treatment workers were accounted for, Pivec said. There were about 60 workers on site at the time, according to The Herald's news partner KIRO-TV, Channel 7.
The injured men were believed to be employees of Apollo Inc., a Kennewick-based general contractor.
The company is investigating the cause of the explosion, Vice President Amy Jenne said Thursday. She declined to release further details until the company learns more.
The families of those injured had been contacted, Jenne said.
"Our concerns are with the people who were injured, and we're doing everything we can to take care of those injured people," she said.
The explosion also is under investigation by the Snohomish County Fire Marshal's Office and the state Department of Labor and Industries.
One neighbor, Joseph Norris, saw construction workers racing down the hill toward the fire after the blast. They were carrying fire extinguishers, he said.
Gary Sabol, who lives nearby, said the powerful blast shook his house.
"I thought it was a tree falling or an earthquake," Sabol said. "Then we saw a plume of smoke, and I expected it was at the plant."
Students at Picnic Point Elementary School, about a mile from the treatment plant, were kept inside.
"Authorities have warned the school to be aware that debris may be in the air near the school," Mukilteo School District spokesman Andy Muntz said. "As a safety precaution, students at the school have been asked to not go outside onto the playfields and will be escorted as they move from building to building."
Some teachers said they believe they heard the explosion from a mile away, Muntz said.
Barbara McMahon was downstairs in her house in the Windandtide subdivision, just up the hill from the plant, when she heard a boom.
"I jumped out of my seat," she said. "It was really loud. I thought maybe my washer had started walking across the floor or something."
McMahon reported that air was filled with a chemical smell.
Sheriff's deputies closed down Picnic Point Road at 140th Street SW for some time. Emergency crews responded from all over south Snohomish County.
Reporters Eric Stevick, Noah Haglund and Winnona Saari contributed to this report.
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