PRESCOTT, Arizona (REUTERS) - Fire investigators in central Arizona launched a probe on Monday into how wind-driven flames closed in on and killed 19 specially trained firemen in a tragedy that marked the greatest loss of life among firefighters in a United States (US) wildland blaze in 80 years.

The precise circumstances surrounding Sunday's deaths of all but one of a 20-member elite "hotshots" firefighting team remained unclear a day after they perished while battling a blaze that has destroyed scores of homes and forced the evacuation of two towns.

But fire officials said the young men fell victim to a volatile mix of erratic winds gusting to gale-force intensity, low humidity, a sweltering heat wave, and thick, drought-parched brush that had not burned in some 40 years.

The doomed firefighters had managed to deploy their personal fire shelters, tent-like safety devices designed to deflect heat and trap breathable air, in a last-ditch effort to survive that ultimately proved futile, officials said.

Mr Peter Andersen, a former local Fire District chief who assisted in the early firefighting efforts, told Reuters some of the men on the ground made it into their shelters and some did not, according to an account relayed by a ranger helicopter crew flying over the area.

"There was nothing they (helicopter crew) could do to get to them," he said.

Still, conditions faced by the "hotshots," a ground crew that fights flames at close range with hand tools and serves as the shock troops in a firefighting force, were typical for the wildfires they are trained to battle, fire officials said.

Standard safety protocols followed by such crews appeared to be in place, and investigators are trying to determine exactly what went wrong in this instance, they said.

"It had to be a perfect storm in order for this to happen.

Their situational awareness and their training was at such a high level that it's unimaginable that this has even happened," Prescott Fire Department spokesman Wade Ward told ABC's Today programme.

STARTED BY LIGHTNING

The blaze was ignited on Friday by lightning near the town of Yarnell, about 128km north-west of Phoenix, and by Monday was still raging unchecked after scorching some 3,400 hectares of tinder-dry chaparral and grasslands.

Authorities ordered the evacuation of Yarnell and the adjoining town of Peeples Valley. The two towns are southwest of Prescott and home to roughly 1,000 people.

A Yavapai County Sheriff's Office spokesman said on Sunday at least 200 structures had been destroyed, most of them in Yarnell, a community consisting largely of retirees. And fire officials said most of the building lost were homes.

Authorities on Monday said that figure was a rough estimate and that a more accurate assessment of property losses was expected later.

The so-called Yarnell Hills blaze was one of dozens of wildfires in several western US states in recent weeks.

Experts have said the current fire season could be one of the worst on record.

Sunday's disaster in Arizona marks the highest death toll among firefighters from a US wildland blaze since 29 men died battling the Griffith Park fire of 1933 in Los Angeles, according to the National Fire Protection Association.

The association lists seven incidents in the United States during the past century that killed as many or more firefighters than on Sunday in Arizona. The costliest saw the deaths of 340 firefighters in the Sept. 11, 2001, attack on the World Trade Center in New York.

Arizona Forestry Commission spokesman Mike Reichling said one member of the 20-man crew had been driving in a separate location and survived unhurt.

Eighteen of the dead were members of the Granite Mountain Hotshots team, assigned to the Prescott Fire Department, and the 19th victim belonged to another crew who was working with the fallen team, Reichling said.

WARNING FROM A BOW HUNTER

Evacuee Rick McKenzie, 53, a bow hunter and ranch caretaker, said the fire had "exploded" on Sunday, with flames 9 to 13 metres racing across an area of oak and brush and that he had warned the Hotshots about the dense oak woods where they would be working.

"I said, 'If this fire sweeps down the mountain to the lower hills where all this thick brush is, it's going to blow up, guys, you need to watch it,'" said Mr McKenzie, who had taken refuge at a Red Cross shelter at Yavapai College.

The Hotshots were highly trained firefighters with rigorous fitness standards. All were required to take an 80-hour critical training course and refresher yearly, according to the group's website.

"Our common bond is our love of hard work and arduous adventure," the website said.

Scorching heat is expected to last for the first part of the week, meteorologists said.

The deaths brought an outpouring of tributes on Sunday from political leaders, including from President Obama, who is on an official trip to Africa.

In a statement, Arizona Governor Jan Brewer called the deaths "one of our state's darkest, most devastating days." She ordered state flags flown at half staff from Monday through Wednesday.

Senator John McCain, an Arizona Republican, said in a statement: "This devastating loss is a reminder of the grave risks our firefighters take every day on our behalf in Arizona and in communities across this nation. Their sacrifice will never be forgotten."