We’re getting more details of yesterday’s explosion at the Alderwood Wastewater treatment plant off Picnic Point Road.

The explosion happened as a 1,000-gallon propane tank was being loaded onto a truck. The tank shifted, a valve snapped and the propane ignited.

Five workers were injured.  Fire District 1 tells us the blast had a 300-foot radius and extended into two nearby buildings.

Investigators believe there was a second explosion inside one building where the injured employees were working. They think flames from the explosion ripped through the building and ignited flammable vapors from a painting process.

All five of the injured workers were admitted to Harborview for serious burns, but one has been released.

Damage is expected to total at least $1 million. This includes damage to the two buildings under construction, building materials, the propane tank and truck, and a construction trailer.

More than 50 firefighters from Fire Districts 1 and 7, Lynnwood, Mukilteo and Snohomish County Airport Fire departments responded.