The Yosemite rockfall, of "undetermined size", took place Wednesday afternoon along the popular Waterfall Route, according to the park.
El Capitan is one of the most popular hiking locations in the country, with the 3,000 vertical feet of granite rock attracting adventure travelers from around the world.
After that, Zabrok said, he and others in his group wrote on Facebook that they witnessed four or five additional rock falls.
Gediman says the two were hiking at the bottom of El Capitan's vertical face in preparation on their way to scale it when the rock fell.
Yosemite National Park is a like a right of passage for avid rock climbers hoping to push the limits of what they can climb.
Authorities say one person was killed and one was injured, and they are searching for more possible victims.
Rockfalls are common in Yosemite Valley, with about 80 each year. "Once we saw how quickly it dissipated, we knew it wasn't a fire". It was unclear if any of the approximately 30 climbers on the rock, or park tourists below, was injured, the Los Angeles Times reported.
"I saw a piece of rock, white granite the size of an apartment building, at least 100 feet by 100 feet, suddenly just come peeling off the wall with no warning", Canadian climber Peter Zabrok, 57, told the AP.
The incident is especially unfortunate as it is now hiking season in Yosemite, according to the park, putting many visitors on the hiking trails in the park.