"Hotshot" crews are elite firefighters who often hike for miles into the wilderness with chain saws and backpacks filled with heavy gear to build lines of protection between people and fires. They remove brush, trees and anything that might burn in the direction of homes and cities.
"They're not the people that have engine companies and large trucks. These are the core of firefighting where they're right there in the middle of the incident," Fraijo said at a Sunday night news conference.
Fraijo said the crew killed in the Yarnell fire worked other wildfires in recent weeks in New Mexico and Arizona.
He added that the firefighters had to deploy the tent-like structures when "something drastic" occurred.
"One of the last fail-safe methods that a firefighter can do under those conditions is literally to dig as much as they can down and cover themselves with a protective -- kind of looks like a foil type, fire-resistant material -- with the desire, the hope, at least, is that the fire will burn over the top of them and they can survive it," Fraijo said.
The Prescott-based crew never had to use shelters during a wildfire, according to a 2012 profile in a Cronkite News Service report.
Reichling, the incident commander, said half of the 500 structures in Yarnell have been destroyed by the "radical weather." Most of Yarnell's 700 residents have been forced to evacuate.
"We have fires throughout the state right now. We have the winds that change. That's what happened today and caused this tragedy," he said.
Evacuee Don Mason said, "I don't know if my home's standing or not, but from what I've heard, most of Yarnell has burned to the ground. So it's been tough."
About 200 firefighters are fighting the wildfire, which has also forced the closure of parts of state Route 89. None of the fire is contained.
President Obama, who is traveling in Africa, released a statement early this morning, calling the 19 firefighters "heroes" for putting "themselves in harm's way to protect the lives and property of fellow citizens they would never meet."
Former U.S. Rep. Gabrielle Giffords expressed her shock on Twitter, calling the death of the firefighters "absolutely devastating news."