The ever-expanding swell of wildfires in Arizona claimed the lives of 19 firefighters on Sunday, as the crew battled plumes of smoke and roaring fires in Yarnell Hill, northwest of Phoenix.

This loss comes as the deadliest day for firefighters since the September 11th attacks and the deadliest wildland fire since 1933, according to a list from the U.S. National Wildfire Coordinating Group.

Authorities believe the wildfire began in Yarnell, about 90 miles northwest of Phoenix, Friday with a lightning strike and spread to at least 2,000 acres Sunday amid triple-digit temperatures, low humidity, and windy conditions, according to ABC news. The 2,000-acre fire had grown three times the size by early Monday morning, Arizona incident commander Mike Reichling stated.

Zero percent of the fire has been contained so far.

The fallen crew was known as a "hotshot" crew, meaning they performed the unique duty of hiking miles into the woods — often with heavy loads on their backs, which included chainsaws and heavy gear weigh upwards of 50 lbs. — to build protection lines between nearby homes and the fire.

"They're not the people that have engine companies and large trucks," said Prescott Fire Chief Dan Fraijo at a news conference Sunday night. "These are the core of firefighting where they're right there in the middle of the incident."

Hotshot crews remove brush, fallen trees, and other hazards that could direct a fire's path towards surrounding houses.

"Our entire crew was lost," said Fraijo. "We just lost 19 of some of the finest people you'll ever meet. Right now, we're in crisis."

Part of the hotshot crew's responsibility includes safely removing any fuel sources that could fall in the fire's path. This often means crew members position themselves dangerously close to the fire itself.

"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," said state forestry spokesman Art Morrison. "Evidently, their safety zone wasn't big enough, and the fire just overtook them."

Fraijo did not offer speculation as to the exact cause of the firefighters' death. The focus of his press conference instead centered on expressing the deep loss felt by the remaining crew, which is now 20 percent smaller.

"We grieve for the family. We grieve for the department. We grieve for the city," he said. "We're devastated. We just lost 19 of some of the finest people you'll ever meet."

PREPARATIONS AND REACTIONS: "THEY WERE HEROES"

In high-intensity situations, a firefighter's last resort — when he or she cannot escape the fire's path, but must face it head-on — is to use a portable fire shelter. The tent-like structure offers the firefighter brief respite from the brutal heat outside and a pocket of breathable air on the inside. Temperatures inside the shelter can reach 200 degrees, while external temperatures can reach as high as 500 degrees.

Lightning sparked the fire on Friday, and gusty winds blew the blaze northwest, ravaging nearby structures and leaving many people wondering if they would have a home to come back to. The majority of the town's 700 residents were forced to evacuate.

"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," said evacuee Don Mason. "So it's been tough."

President Barack Obama, who is travelling in Africa, offered a written statement praising the firefighters' selfless acts of courage.

"They were heroes—," he wrote, "highly skilled professionals who, like so many across our country do every day, selflessly put themselves in harm's way to protect the lives and property of fellow citizens they would never meet."

CNN reports that Arizona Governor Jan Brewer is slated to visit Prescott on Monday. She offered her condolences and commitment to suppressing the fire's continued destruction and spread.

"It may be days or longer before an investigation reveals how this tragedy occurred, but the essence we already know in our hearts: fighting fires is dangerous work," she said.

"When a tragedy like this strikes, all we can do is offer our eternal gratitude to the fallen, and prayers for the families and friends left behind."