The Feb. 23 explosion sent fire ripping through the construction site at the Alderwood Water and Wastewater District plant along Picnic Point Road, between Edmonds and Mukilteo.
The fire seriously injured five people and caused more than $1 million in damage.
Snohomish County fire investigators determined the explosion happened while a crane was moving a 1,000-gallon propane tank. The weight of the tank somehow shifted, shearing off a valve and exposing about 300 gallons of propane.
The late-morning explosion alarmed neighbors and sent emergency vehicles pouring in from all over the county.
Because the explosion involved serious injuries at a workplace, the state Department of Labor and Industries was required to investigate whether safety laws had been followed.
The probe found no violations by the general contractor, Kennewick-based Apollo Inc., L&I spokeswoman Elaine Fischer said last week.
The L&I investigation continues, however, into a subcontractor at the site, propane company Ferrellgas. Injury-related investigations generally must wrap up within six months of an incident.
The treatment plant has been under construction since 2008 to double the site's sewage-treatment capacity.
Damage from the fire has delayed construction, said Mike Pivec, water district spokesman.
The plant still can handle liquid waste, but the fireball tore through the solids-handling facility, damaging machinery and electronic equipment inside, Pivec said.
The operations building had lesser damage from heat exposure.
The water district hopes to move people out of trailers at the site and into offices in the coming weeks.
The project is scheduled for completion in the fall.
Rikki King: 425-339-3449; firstname.lastname@example.org
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