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Two climbers who plunged to their deaths from El Capitan were using a risky technique known as simul-climbing to increase their speed and may have neglected to attach their rope to an anchor or any safety device that could have prevented the first falling climber from pulling his partner off the cliff, said a witness and climbing experts.

Best friends and longtime climbing partners Tim Klein, 42, of Palmdale (Los Angeles County) and Jason Wells, 46, of Boulder, Colo., fell about 1,000 feet June 2 from what experts said was the easiest section of a route called Freeblast, which they had climbed many times before.

It is impossible to know with any certainty exactly what happened, but one eyewitness and several climbing experts familiar with the route and the techniques that are normally used on it have pieced together the most likely scenario — and it paints a grim portrait of possible overconfidence, complacency, miscommunication or all three.

The death-defying adventure of scaling a huge granite precipice like El Capitan involves an intricate process in which lengths of nylon rope are tied to the climbers and knotted on bolts, or anchors, at the beginning of each section or pitch of the climb, which can be as long as 150 feet. The standard method is for the lead climber to attach the rope to safety gear inserted in cracks in the rock at intervals, so that he can only fall as far as the last piece of gear he placed if he slips. The bottom climber belays — controlling slack in the rope and arresting falls — and then climbs up after the lead climber finishes the pitch.

In simul-climbing, climbers scale the wall at the same time with the bottom climber belaying as he goes. The lead climber is still supposed to place safety gear in cracks in the event of a fall, but the rope is not anchored at the bottom as is standard. This is what climber Jordan Cannon said he saw Klein and Wells doing minutes before they fell.

The pair, who were reportedly training for a speed ascent of the Salathe Wall route on El Capitan, were approaching a feature called Mammoth Terraces at 8:15 a.m. on the last and easiest of the 10 pitches on Freeblast when the accident happened. They had taken with them a third climber, Kevin Prince, but he was clipped into a separate rope in a rock alcove and told friends he didn’t see what happened.

Cannon, 24, of Reno, who was at the start of a five-day trek up a different route that begins on Freeblast, told The Chronicle he had climbed with the men for several pitches before letting them pass. He and his partner were about 200 feet below at the time of the accident.

“I remember hearing a yell or a scream of some sort, and then I heard something start to fall and my first thought was that it was a haul bag,” said Cannon, who was underneath a rock outcropping, known as a roof, and could only see out to his left.

Suddenly, Wells flew by, violently bouncing, rolling and sliding down the granite face, he said. As frightening as that was, Cannon expected his fall to be arrested by Klein, who, had they not been simul-climbing, would have been tied into a fixed anchor bolt at the bottom of the pitch they were climbing.

Instead he heard Klein yell “Oh f—” as the rope pulled him off the wall. A split second later he too whooshed through the air, still attached to the rope. Their free fall was halted for a split second when the rope got snagged on a granite flake or block, but the force from two falling bodies was too much.

“The rope exploded,” Cannon said. “It was a blue rope, and we could see the white core strands fly out.”

The two climbers never had a chance, falling from a height almost equivalent to the Empire State Building.

“It was very traumatic,” Cannon said.

Cannon said he had noticed while climbing with the two men that Wells, the leader, was placing very little gear, “less than normal for a good climber.” On one pitch, a more difficult section than the one they fell off, Wells did not place any protection at all.

“There is a point in which it is cavalier to not place gear. It was slightly worrisome,” Cannon said. “That’s because I’ve fallen while speed climbing before, but he looked very solid so it didn’t give me much pause.”

Photo: Greg Murphy 2017

A Yosemite Search and Rescue crew is investigating the accident, but climbing experts familiar with the case suspect, given the lack of protection Cannon described, that the pair did not place gear or set an anchor belay. In that case, either one of them would have pulled the other off with a slip or lost hold.

Cannon later climbed up to the place where the accident occurred to help Prince, who, like himself, was traumatized, and saw rope abrasions on the rock, but could not find any remnant equipment that would indicate a safety system failure.

Photo: Courtesy Abraham Shreve

“Because the rope was severed in the fall, we can’t know whether they placed gear or not ... but because they fell, if they had put gear in it wasn’t very good,” said Hans Florine, the foremost expert on simul-climbing. “In roped climbing usually somebody is anchored, so the person who falls is the only damaged person, but in this case both climbers got taken out, which is unusual.”

The horrific deaths of the two friends — who regularly climbed the hardest multiday routes on El Capitan in a day — were the latest in a troubling series of accidents that have led many to think hard about the climbing community’s recent obsession with speed and risk taking.

The simul-climbing technique Klein and Wells were using was popularized by sponsored climbers like Alex Honnold and Tommy Caldwell, who on Wednesday climbed the Nose of El Capitan in less than two hours, breaking their own speed record.

Ironically, a crowd of spectators, many with binoculars, had gathered in El Cap Meadow that morning to watch Caldwell and Honnold attempt the record. They were instead exposed to tragedy.

“It’s pretty obvious to me they didn’t have their full focus,” said Ken Yager, founder and president of the Yosemite Climbing Association and a longtime climbing guide, who believes Klein and Wells were so confident on the section from which they fell that they violated one of the cardinal rules of climbing with a partner — always place protection or tie into an anchor.

Yager said the accident illustrates a growing problem in Yosemite and other premier climbing spots where enthusiasts, backed by advertising and sponsorships, often are more interested in fast climbing and spine-tingling risks than the transcendental feeling one gets on a wilderness excursion.

The lure of sponsorship money is pushing the masters of the sport to put themselves at greater peril, he said. A case in point, at least where risk is concerned, is Honnold, who last June climbed a route known as Freerider, which includes the area where the recent accident occurred, without any ropes or protection.

Honnold spent three hours and 59 minutes scaling the nearly vertical 3,000-foot granite face knowing that a single slip, missed handhold or unforeseen incident would result in certain, gruesome death. He is among a long line of Yosemite daredevils who have deals with equipment manufacturers or advertisers that market images of them scaling dangerous-looking rock faces.

“These guys have gotten physically more fit and are pushing limits. Back in my day it was us and the rock. You had no distractions,” Yager said. “Nowadays the climbers are posting on social media, there is a lot of interaction with crowds in the valley, and these professional climbers are dependent on doing something new all the time so they can keep their sponsorships and make money. It’s a very dangerous game.”

The concern is not just for people like Honnold — whose nerve and skill borders on superhuman — but for the young climbers who inevitably try to follow in their footsteps.

“The part that bothers me the most is the younger guys who see these guys and try to emulate them,” said Charles “Butch” Farabee, of Tuscon, a former Yosemite ranger who co-wrote the book “Off the Wall: Death in Yosemite,” and keeps a database of all 1,839 deaths that have occurred in the park. “I’ve never had any issue with people climbing, the part that concerns me is putting the rescuers at risk. Somebody has to go up and clean up the mess.”

The warning signs have been mounting over the past few years. Five people have died in climbing accidents on El Capitan since 2013. In all, Farabee said, 31 of the 120 climbers who have died in the park since 1905 have died on the famous granite monolith. He said 23 were killed in falls, including a rescuer who rappelled off his rope, two were hit by rockfalls and six people died from hypothermia.

The deaths of Wells and Klein marked the fourth multiple death on El Capitan since 1978, when three climbers fell more than 1,000 feet after their anchor broke.

Farabee’s casualty list doesn’t include Quinn Brett, one of the most accomplished female climbers in the world, who was paralyzed in October in a 100-foot fall.

Even Florine, who wrote the book on speed climbing and often repeats the mantra “safety first,” broke bones during a fall last month. He admitted this week that he has occasionally climbed easy terrain without a rope, but said he always places protection when he is climbing with someone else.

The route Wells and Klein were on leads into Salathe Wall, which they told Cannon they were planning to complete that fateful day before climbing the Nose the next day.

“Not many people do that, so I thought, ‘Whoa, they’re the real deal,’” said Cannon, adding that they were such nice, friendly guys that he hoped to see them again in the valley. “They were just out to have a good time on a route they were familiar with. A lesson you hear a lot is never let your guard down. ... You can’t predict the things that will happen to you.”

Peter Fimrite is a San Francisco Chronicle staff writer. Email: pfimrite@sfchronicle.com Twitter: @pfimrite