An elite team of 19 firefighters died Sunday while trying to protect themselves under fire-resistant shields from a fast-moving wildfire in Yarnell, Ariz., according to officials.

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. As of 3 a.m. this morning, the Yarnell fire had tripled in size and is now 6,000 acres, according to Arizona incident commander Mike Reichling.

Eighteen of the victims were from the Prescott Granite Mountain Hotshots, fire officials said. It's unclear where the other victim was based.