An accidental explosion at a wastewater facility near Edmonds on Thursday caused $1 million in damage to two buildings under construction.

The Snohomish County Fire Marshal's Office said the fire and explosion at the Alderwood Wastewater facility at 6315 Picnic Point Road was reported by workers there at 11 a.m.

Fire officials said propane vapors ignited while a contractor was lifting a 1,000-gallon propane tank onto a truck to remove it from the property.

• Slideshow: Blast leaves truck charred, twisted

As the tank, which had about 300 gallons of propane inside, was being placed onto the metal deck of the truck, a brass drain valve on its bottom broke off, releasing propane from under the tank.

It was estimated that it took about 45 seconds for the vapors to be ignited.

Snohomish County Fire District 1 was the first fire agency to arrive at the scene and crews triaged about 40 of the 60 employees at the work site and transported five men with burn injuries to Harborview Medical Center.

In addition to the damaged buidlings, a truck was destroyed in the blast.