Five construction workers were burned Thursday when a propane tank exploded at Picnic Point Wastewater Treatment Plant.

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A propane tank exploded at a wastewater-treatment plant Thursday, burning five workers when a fireball ripped through a building they were constructing at Picnic Point Wastewater Treatment Plant.

One worker was in serious condition and three others were in satisfactory condition at Harborview Medical Center. One was treated and released.

The accident happened just after 11 a.m. at the treatment plant, which is being rebuilt and expanded north of Edmonds on Picnic Point Road.

A worker was putting a 1,000 gallon propane tank onto a truck to take it off the site. The tank, which was the size of a small car and had been used temporarily for heat, had about 300 gallons of propane in it.

The load shifted, causing the tank to move unexpectedly, and a valve snapped off the tank, said Leslie Hynes, a spokeswoman for Snohomish County Fire District 1. The truck driver “yelled for people to clear the area, and then the explosion occurred,” she said.

The five victims, all men, were ages 31 to 48, and all had upper-body burns, said Harborview spokeswoman Susan Gregg.

They work for Apollo Construction, which is doubling the capacity of the treatment plant — a $90 million project. It’s scheduled to open late this year. Two buildings are under construction, and all of the injured were in the larger of the two, Hynes said. The truck driver was not injured. Fire inspectors believe there was a second blast, when fire from the first one ignited paint vapors inside the building.

The site will be closed through the weekend while Apollo and other investigators try to find out what went wrong, said Dan Briscoe, an Apollo vice president.

“This was a serious accident. This is something we are taking extremely seriously,” he said. “Five of our workers went to the hospital. That’s unacceptable to us.”

Apollo, based in Kennewick, sent a news release this week about its safety record, which is among the best in the state, according to the state Department of Labor and Industries. The state issues a score based on how long a company has worked without any injuries, and Apollo has the lowest score of any private-sector employer.

A subcontractor from the Puyallup office of Ferrellgas was picking up the propane tank when the explosion occurred. A Ferrellgas representative said he didn’t have any information about the accident.

In addition to the two buildings, the fire involved the truck and a portion of an adjacent wooded area, but was quickly contained, Hynes said. The burned-out truck sat outside the partially constructed building after the explosion Thursday, and workers in hard hats milled around with firefighters and investigators.

Shawn Kiesling was working nearby and saw flames shooting out the windows of the building next to the propane tank.

“It was pretty scary, probably the biggest fireball I think I’ve ever seen,” he said.

The treatment plant is being rebuilt by the Alderwood Water and Wastewater District, and is nearing completion. About 60 workers were on-site at the time of the explosion, Hynes said.

The blast caused at least $1 million damage to the structures being built and other equipment, she said.

Students at Picnic Point Elementary, about a mile away, were kept inside for a time in case toxic chemicals were in the air, a school spokeswoman said. Some teachers on one wing of the school reported hearing the explosion, she said.

Emily Heffter: 206-464-8246 or On Twitter @EmilyHeffter.