A climber who was above a deadly rockfall at Yosemite National Park says a hunk of granite “the size of an apartment building” fell off the mountainside.

Peter Zaybrok of Ontario, Canada, says he was on his sixth day of a climb up the popular El Capitan on Wednesday when he looked down and saw the rockfall.

El Capitan is one of the world’s largest granite monoliths towering vertically 1,219 meters from the Yosemite Valley floor.

Authorities say one person was killed and one was injured, and they are searching for more possible victims.

Zaybrok says he would be dead if he’d been climbing at his usual leisurely pace.

The 57-year-old says that he doesn’t know how anyone who was below the rockfall could have survived.

Park Ranger Scott Gediman says rocks came crashing down on Wednesday at the height of climbing season with at least 30 climbers on the wall.

Gediman says the injured person was being taken to a hospital near the park. No identities were released.

Gediman said on Wednesday that witnesses made multiple calls reporting the rockfall around 2pm (local time). He estimates 30 or more climbers were on the wall at the time, but he could not confirm injuries or the size of the rockfall at the time.

Mountaineers worldwide travel to the park to scale the sheer face.

The rockfall happened along the Waterfall route on El Capitan’s eastern buttress.