KATMANDU, Nepal (AP) – An American climber was killed on Mount Everest after he slipped and fell into a crevice, Nepalese mountaineering officials said Tuesday.

Seattle climber Michael O’Brien, 39, fell to his death on Sunday as he and his brother Chris, 32, were returning to their base camp and were crossing the Khumbu Icefall, a dreaded section of the route that has claimed the lives of many climbers.

“I honestly never even dreamed anything bad would happen to them. We’re all numb and shocked,” his oldest sister, Kathryn Caltabiano, told The Seattle Times. “Mike was physically fit, mentally stable and cognitively aware of the challenge.”

Climbers say the Khumbu Icefall looks like a mass of huge ice cubes spread across the mountain. Freezing temperatures at night and the warm sun during the day make the ice unstable and unpredictable. Climbers use lightweight aluminum ladders tied together by ropes as bridges to cross over ice crevices as deep as 100 feet.

According to the O’Briens’ Web site, they were attempting to become the first American brothers together to reach the summit of Mount Everest, the world’s highest mountain.

“Our intent is to raise money for the Hereditary Disease Foundation, which funds scientific research seeking cures for diseases such as Huntington’s, Alzheimer’s, and Parkinson’s,” the brothers said on their Web site.