The state of Arizona is mourning the loss of 19 firefighters who died Sunday battling a wildfire northwest of Phoenix, CNN reports. The firefighters were part of an elite squad who confronted wildfires up close, setting up barriers to stop the spreading destruction. It was the deadliest day for firefighters since 9/11, and it is the deadliest wildland fire since 1933, according to a list from the U.S. National Wildfire Coordinating Group. "Our entire crew was lost," Prescott fire chief Dan Fraijo said Sunday night. "We just lost 19 of some of the finest people you'll ever meet. Right now, we're in crisis." Fraijo said he did not know the exact circumstances surrounding the firefighters' deaths and wouldn't speculate on a cause, but he said drought conditions, combined with winds that whipped unpredictably, have made battling the flames especially difficult. 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, incident commander Mike Reichling said. Department of Homeland Security Secretary Janet Napolitano urged residents in the area to heed local authorities' instructions, while lamenting the loss of so many firefighters. President Barack Obama also lauded the efforts of the fallen firefighters, saying their deaths are heartbreaking and "our thoughts and prayers go out" to their families.