(CNN) -- They were part of an elite squad confronting wildfires on the front line, setting up barriers to stop the spreading destruction. But in their unpredictable world, it doesn't take much to turn a situation dangerously deadly.
In this case, a wind shift and other factors caused a central Arizona fire, which now spans more than 8,000 acres, to become erratic, said Mike Reichling, Arizona State Forestry Division spokesman.
The inferno proved too much, even for the shelters the 19 firefighters carried as a last-ditch survival tool.
"The fuels were very dry, the relative humidity was low, the wind was coming out of the south. It turned around on us because of monsoon action," Reichling told CNN affiliate KNXV. "That's what caused the deaths."
The firefighters were killed Sunday while fighting the Yarnell Hill fire, northwest of Phoenix. Among the dead was Eric Marsh, superintendent of the Granite Mountain Hotshots, according to his father, John Marsh.
The families of the 18 other firefighters have been notified. There are no other reported injuries from the blaze, Reichling said.
"As we face the day the highest priority is for the fallen comrades," said Roy Hall, an incident commander with the state forestry division. "We got a lot of hotshot crews in the nation, and they are the elite of the ground firefighters. They're highly trained and highly specialized. They are a younger generation. That's the tragedy of it, that lives would be lost of such a young group."
He added of the fire, "We know that there are values to be protected and efforts that need to be ongoing in this fire. It's a long ways from being over."
Firefighters will work on the eastern side of the fire in an effort to protect homes in evacuated areas, as air tankers drop fire retardant on the perimeter and five helicopter douse hot spots with water.
Temperature could soar as high as 102 degrees, and with a chance of thunderstorms, the fire's behavior could again become erratic because of the gusts, high temperatures and dry fuel in the fire's path.
Involved in the firefighting effort are 18 engines, 18 fire crews, two structure-protection vehicles and four bulldozers. Commanders have ordered more hotshot crews, firefighting personnel and equipment.
Five members of the New York Fire Department incident management team are being sent to assist with management, logistics and strategy, FDNY spokesman Frank Dwyer said.
Department loses 1/5 of squad
It was the deadliest day for firefighters since the 9/11 attacks. And it is the deadliest wildland fire since 1933, according to a list from the U.S. National Wildfire Coordinating Group. Twenty-five firefighters died when a blaze burned in light chaparral near Griffith Park, California.
"Our entire crew was lost," Prescott Fire Chief Dan Fraijo told reporters Sunday night. "We just lost 19 of some of the finest people you'll ever meet. Right now, we're in crisis."
The tragedy killed about 20% of the Prescott Fire Department. Fraijo said one member of the team was not with the other crew members and survived.
Authorities have information that during the blaze, the firefighters deployed their fire shelters, a sort of aluminum blanket that protects against the flames and heat.
The shelters must be timed well. Set it up too soon, and the heat inside the shelter can become suffocating. Deploy it too late, and the fire is already on top of you.
Wearing gloves, a firefighter will lay on the ground under the shelter, the ground being the only thing keeping the firefighter cool. The shelter will block 100% of the heat from flames and hot gases and 95% of the radiant heat from the flames themselves.
Authorities believe lightning sparked the Yarnell Hill fire on Friday. By Sunday night, it had scorched more than 6,000 acres and destroyed more than 100 structures, Reichling said.
Billows of thick black smoke covered the sky as the giant flames leaped from one stretch of parched land to another.
The wildfire also forced evacuations in Peeples Valley and Yarnell, but no civilian injuries were reported.
Drivers fleeing the area were chased by dark plumes filling the air. Some evacuees paused to look from afar, wondering if the flames had torched their homes.
The blaze hadn't touched Prescott yet. But like many other fire departments across the state, the Prescott team jumped in to help.
"A hotshot crew are the elite firefighters," state forestry spokesman Art Morrison said. "They're usually (a) 20-person crew, and they're the ones who actually go in and dig the fire line, cut the brush to make a fuel break. And so they would be as close to the fire as they felt they safely could."
"In normal circumstances, when you're digging fire line, you make sure you have a good escape route, and you have a safety zone set up," Morrison said. "Evidently, their safety zone wasn't big enough, and the fire just overtook them."
Fraijo, the fire chief, said he did not know the exact circumstances surrounding the firefighters' deaths