YARNELL, Arizona- Varying winds caused by rugged terrain and Arizona's monsoon may have contributed to the deaths of 19 firefighters over the weekend.

The firefighters, part of the elite Granite Mountain Hotshots based in nearby Prescott, were killed Sunday evening while fighting an erratic, lightning-sparked fire in the mountains about 85 miles northwest of Phoenix.

Fire officials aren't sure how the firefighters became trapped, but outflow boundaries from thunderstorms and a channeling effect caused by the topography of the mountains in the area can cause varying wind speeds and directions during Arizona's monsoon.

The early part of Arizona's monsoon season, which began on June 15, is the most dangerous time for fires because the storms often produce plenty of lightning and wind, but not much rain.

JOHN MARSHALL, Associated Press