YOSEMITE NATIONAL PARK, Calif. (AP) — A Stanford University graduate student has died while climbing in Yosemite National Park.

Friends say 31-year-old Christina Chan was descending a granite peak known as Eichorn Pinnacle on Friday afternoon when she fell several hundred feet to her death. It’s not clear what caused the fall, but officials say Chan was climbing without ropes or safety protection.

Friends say she was an experienced climber who had scaled Yosemite’s El Capitan several times and knew the risks of climbing. She taught rock climbing as part of the Stanford Alpine Club.

Chan was pursuing her doctorate in political science at Stanford.