And in 2015, Tyler Gordon fell to his death while climbing the Nose.

Mr. Griebel said Mr. Klein had climbed El Capitan — known as El Cap among climbers — more than 100 times. Mr. Wells had also climbed it many times, he said.

“It’s safe to say they knew exactly what they were doing,” he said.

Brady Robinson, the executive director of the Access Fund, an organization that seeks to protect climbing areas in the United States, said Mr. Wells was one of his best friends, and they often climbed together on routes in Boulder.

El Capitan is “much bigger than anything around here, which is why he liked it,” Mr. Robinson said. “What he used to do was he would fly out on a Friday, climb El Cap twice — once on Saturday and once on Sunday.”

“That is almost unheard-of,” he added. Mr. Wells was “just one of these undercover world-class athletes that almost nobody knew about.”

Mr. Robinson said Mr. Wells and Mr. Klein were using a technique called simul-climbing in which both climbers are attached by a rope and move at the same time to go at a faster pace. They were doing this with a third person, a variation on an already rare technique that is “inherently riskier” than regular climbing, Mr. Robinson said.

The third person, who was not identified by the National Park Service, was on a separate rope and anchor, and not securely attached to the same system being used by Mr. Wells or Mr. Klein, according to Mr. Robinson, and was unharmed.