In this Monday, Nov. 15, 2010 photo provided by Tomas Ovalle, extreme athlete Dean Potter stands in front of El Capitan after a speed climbing attempt up El Capitan in Yosemite National Park, Calif. Potter, renowned for his daring and sometimes rogue climbs and BASE jumps, and his climbing partner Graham Hunt were killed Saturday, May 16, 2015 after jumping from a 7,500-foot promontory called Taft Point in Yosemite National Park. (AP Photo/Tomas Ovalle)

SAN FRANCISCO - Two wingsuit fliers who leaped from a cliff in Yosemite National Park were trying to zoom through a notch in a ridgeline and were airborne for about 15 seconds when they slammed into a rocky outcropping and were killed, a friend said Monday.

Dean Potter, 43, and Graham Hunt, 29, were experienced at flying in wingsuits — the most extreme form of BASE jumping, a sport so dangerous enthusiasts keep lists of the dead.

On Saturday, wearing the "flying squirrel" outfits that have fabric stitched between the arms and body and between the legs to keep them aloft, Potter and Hunt leaped off Taft Point, 3,500 feet (1,050 metres) above the valley floor. They would have been travelling at speeds close to 100 mph (160 kph) as they aimed for the narrow gap in the ridge.

Their bodies were found in a notch they had already flown through about a dozen times, professional climber Alex Honnold said. No one knows exactly what went wrong. A gust of wind or a slight miscalculation could have sent them off course, hurtling into rock.

"What they were doing is pretty routine" for them, Honnold said. "Not like a once-in-a-lifetime performance."

BASE jumping — renegade parachuting off buildings, antennas, spans (such as bridges) and Earth (in this case, the cliffs over Yosemite Valley) — is illegal in national parks. Doing it in a wingsuit is even more dangerous, particularly the form Potter practiced, gliding frighteningly close to cliffs and trees before deploying his chute.

Potter was "the big inspiration to the climbing community in the last generation," and Hunt "was maybe the most prolific BASE jumper in the valley right now," said Honnold, whose own free-solo ascents of America's biggest cliffs have made him one of the world's most recognized climbers.

But he has never BASE jumped, and he has no desire to try.

Hunt grew up in the Northern California town of Shingle Springs.

Potter, who was originally from New Hampshire and most recently lived near Yosemite, thought he had found ways of safely enjoying some of the world's riskiest endeavours. He scaled the toughest vertical faces without rope, and walked barefoot across lines suspended between cliffs. If he fell or became exhausted, he would deploy a parachute.

As if that didn't provide enough adrenaline, Potter wanted to fly.

"Many people consider me crazy for my innovations and changes to climbing saying I'm reckless and taking too many risks," Potter wrote three weeks ago on Instagram, where he posted a photo of himself leaping from a cliff in Switzerland in 2009 and transforming "dying into flying."

"I'm still injury free after over 30-years of pursuing some of the most dangerous #OutdoorArts known to man, #KnockOnWood and will continue to take the necessary precautions to stack the odds in my favour in order to live a long and happy life," Potter wrote. "There is a way to play hard and stay safe!"

In "Fly or Die," a documentary about his wingsuit jumps that can be seen on National Geographic's website, he said: "Everyone kinda fantasizes about it — flying. And it's an amazing place in history right now, that man actually has the skills to pull it off."

At least five people have died in BASE jumping accidents in national parks since January 2014, including the most recent deaths at Yosemite, said Jeffrey Olson, a National Park Service spokesman. Two of those were at Utah's Zion and one at Glacier in Montana.

The park service celebrates Yosemite's role as a climbing mecca, but it struggles to stop people from illegally leaping off the cliffs. Jumpers who are caught get fined and see their equipment confiscated, but Honnold compared that to speeding tickets for race-car drivers.

"For them, BASE jumping was like their art, their passion," and they faced a much more powerful repercussion in any case, he said: "The potential down side of base jumping is dying."

In 2009, Potter set a record for completing the longest BASE jump, from the Eiger North Face in Switzerland, by staying in flight in a wingsuit for 2 minutes and 50 seconds. The feat earned him the Adventurer of the Year title by National Geographic magazine.

Potter's solo ascent of Utah's iconic Delicate Arch in May 2006 prompted the National Park Service to ban climbing any of the named arches or natural bridges in Arches National Park. The outdoor clothing company Patagonia stopped sponsoring him, saying his actions "compromised access to wild places and generated an inordinate amount of negativity in the climbing community and beyond."

Clif Bar also withdrew its sponsorship of Potter for taking risks it couldn't support. But Potter held onto Adidas and other sponsors, even after he packed his miniature Australian cattle dog, Whisper, on his back for some of his jumps and was criticized by animal rights groups. The dog was not with him on Saturday's fatal jump.

"Dean Potter was an inspiration for many of us. He was an innovator and pioneer, always seeking for new creative solutions, an exceptional athlete and artist, who loved what he was doing," Adidas said in a statement. "We lost a friend. You will be deeply missed Dean."

Another sponsor was Five Ten footwear, whose spokeswoman, Nancy Bouchard, said "Dean would have pursued these activities whether he was supported or not. In the back of our minds, we always knew something terrible could happen, but that didn't and doesn't diminish our feelings."

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Smith reported from Fresno. Daisy Nguyen and Brian Melley contributed from Los Angeles.

News from © The Associated Press, 2015