Five men were injured in an explosion at a treatment plant under construction in Edmonds.

The explosion occurred at the Alderwood Water and Wastewater District plant in the 6300 block of Picnic Point Road late Thursday morning.

Snohomish County Fire District 1 said the injured men, who are workers with Apollo Construction, were transported to Harborview with burns to their upper bodies. 

• Slideshow: Blast leaves truck charred

Workers were loading a 1,000-gallon propane tank onto a truck when a valve may have been sheared off, causing an explosion that sent a fireball through a building under construction.  The tank was 30 percent full at the time of the blast.

"I ran outside to take a look, and there were people running, construction workers were running," said Joseph Norris, who lives nearby. "It wasn't a loud firecracker pop, this was -- it sounded like a bomb went off."

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The injured men, who range in age from 31 to 48, were in that building when the explosion occurred.  Four of the men are in satisfactory condition, and a fifth man is in serious condition in the intensive care unit, but is expected to be OK.

Video from Chopper 7 showed a Ford flatbed truck that was badly burned and twisted, but the tank on the back of the truck appeared to be unharmed.

The plant has been under construction since 2009 and was scheduled to be finished in November.

Officials said 60 workers are at the site.