EDMONDS, Wash. -- When a propane tank exploded at a construction site near Edmonds last week, Snohomish County firefighters and paramedics were ready.
Crews were summoned about 11 a.m. Thursday to the Picnic Point Wastewater Treatment plant project. A valve had sheared off the tank, exposing 300 gallons of propane that then ignited.
Initial reports said as many as 20 people were injured. Flames were spreading into nearby buildings and trees.
The 911 calls set in motion a sophisticated plan for dealing with a large-scale emergency, said Capt. Shaughn Maxwell, who helps oversee medical operations for Snohomish County Fire District 1.
Within 15 minutes of crews arriving on scene, the three most critically injured people were in ambulances on their way to the region's trauma center in Seattle, Maxwell said. Within 22 minutes, two more were being transported, and roughly 45 others had been medically evaluated.
Thursday's incident had the potential to become what emergency responders call a "mass casualty incident" -- a crisis that overwhelms available resources. This time, there was a fire, at least two explosions and a large number of people who reportedly had been hurt.
Fire District 1 only sees an emergency of that scale every few years, but they have to be ready, Maxwell said.
"It doesn't happen a lot, but it does happen for sure," he said.
Every firefighter and medic in the district is drilled at least once a year on a mass-casualty scenario, he said. Real-life calls in the past have included carbon monoxide poisonings, pepper spray going off in a crowded building, and a track team attacked by stinging insects.
The fire district has been able to take advantage of post-9/11 Homeland Security funding and training, Maxwell said. They use emergency-response models developed in war-torn countries, where crews are accustomed to mass trauma.
On Thursday, emergency responders relied on a comprehensive county-wide plan that includes constant drilling, Maxwell said.
When practicing for mass-casualty events, Fire District 1 drills with neighboring fire departments, including the one at Paine Field, in addition to private ambulance companies, he said.
"All these agencies drill together so they can come together when these incidents happen," he said.
The planning and drilling gives them a streamlined framework for tackling a chaotic event, he said.
Every crew that responds to a big emergency is assigned a specific role. That might include a group to get people to hospitals, a group that focuses on rescuing anyone trapped and a group to put out a fire.
"Our first issue is life, life-safety, and then fire suppression," Maxwell said.
A number of factors came together Thursday that helped the emergency response work as well as it did, Maxwell said.
To start with, there were few other calls for help in the area at the same time. Fire departments from Lynnwood, Mukilteo, Clearview and Paine Field shared people and equipment.
Rural/Metro, a private ambulance company, pitched in from its fleet.
"In a normal situation, you might set up a treatment area and start treating those people waiting for ambulances," Maxwell said. "In this situation, everything went right, and we were able to immediately load every patient into an ambulance and start transporting them."
All of those injured had been working for a general contractor at the site, officials said. They are all expected to recover from the burns they received.
The explosion has been ruled an accident. Because it involved serious injuries in a workplace, it remains under investigation by the state Department of Labor & Industries.
Construction at the site remained halted Monday, said Mike Pivec, an administrative services manager with the Alderwood Water and Wastewater District.
District officials hope to determine in the next few days when work can resume, Pivec said. Sewage treatment at the plant has not been impacted.