When Geoffrey Farrar was found bleeding from the head and lying mortally injured at the base of a cliff near the Billy Goat Trail in Montgomery County, word quickly spread through the rock-climbing community that the 69-year-old had taken a rare, tragic fall.

The Arlington County retiree was such a fixture at the Potomac area’s Carderock Recreation Area that people called him “Carderock Geoff.” He was known for dispensing advice and taking chances. At the recreation area, part of the Chesapeake & Ohio Canal National Historical Park, he scaled sheer rock faces looming over the Potomac River, usually without safety ropes.

But his death on a Saturday afternoon, three days after Christmas, was no climbing accident, according to federal authorities. They allege that he was killed in a dispute with a younger climber he had mentored for about two decades and that the weapon was a claw hammer, a tool common in the sport, wielded near one of the C&O Canal park’s most popular trails.

The U.S. attorney’s office charged David DiPaolo, 31, of Bristow, Va., with manslaughter after his arrest Jan. 8 near Glens Falls, N.Y. He has agreed to be returned to Maryland to stand trial, authorities said. Court documents quote him telling police that he acted in self-defense while being choked during an argument with his friend.

The incident has caused a stir among rock-climbing enthusiasts who took to the Internet to post notes of condolence and personal remembrances. If Farrar was the most recognizable climber at Carderock, people who knew him said, his sidekick DiPaolo, or “Carderock Dave,” was a close second.

Farrar had been climbing rocks “probably longer than I was alive,” said William Kelley, 28, of the District, an instructor and guide at Earth Treks, a climbing center based in Howard County. Kelley took students to Carderock and knew both Farrar and DiPaolo from seeing them there.

Kelley said that “every time you saw [Farrar] he was in a conversation with someone, giving them information about climbs. He was the guy at Carderock. He knew all the climbs, and he knew how they were supposed to be done.”

Farrar’s relatives could not be reached for comment. A condolence page on the Web site of the Randolph Funeral Home in West Virginia said a memorial service would be announced at a later time. Notes were posted by friends who said they “bouldered” with Farrar hundreds of times and by others who said he helped them through their first climbs. His daughter, Sharon Osman, wrote of him that “you died doing something that you loved to do.” A friend posted, “He made Carderock a welcoming place.” Another said, “You are taking your highest climb, my brother.”

Pictures that have surfaced of DiPaolo show long scraggly hair and a rough beard. His dress style could be described as grunge, and he is known for wearing mismatched shoes.

DiPaolo’s father, Vincent, 65, said he and his son met Farrar at Carderock 20 years ago when the then-11-year-old boy got interested in climbing. Farrar and David DiPaolo formed a strong bond. Farrar set the pace on increasingly challenging climbs, Vincent DiPaolo said, ordering the younger climber to follow his precise path up a rock face, sometimes using only the smallest toeholds.

Later, they competed on dangerous climbs. “It was a game they played,” Vincent DiPaolo said by phone from Bristow. “They climbed without ropes. They liked to dare each other.”

He said his son has bipolar disorder and dabbled in surrealist art. He described Farrar as demanding and tough — and a superb climber. “I feel so bad about the whole thing. I feel awful,” he said. “Both had admiration for each other. At the same time, there was always a competition.”

Vincent DiPaolo said of his son: “He’s no murderer. If he had to defend himself, then, yeah, he would defend himself.”

Kelley said Farrar had told him that he and David Dipaolo had a dispute during a climb in West Virginia, with Farrar blaming DiPaolo for mistakes that caused another climber to fall off a safety line. After that, he said, DiPaolo rarely returned to Carderock.

But authorities said the two were there with other friends Dec. 28. They were seen arguing in the parking lot before starting a climb. The court documents state that two other climbers went to the top of the rock face to secure ropes but that when they looked over the edge, they didn’t see anyone. They walked to the bottom and saw DiPaolo running up the trail, wearing a green hooded sweatshirt, a white and black shoe and carrying an orange camouflage backpack, according to the court documents.

Police said the two friends found Farrar lying on the trail next to a wooden beam, bleeding from the head. The court documents said an autopsy performed by the Maryland medical examiner’s office found that the injuries were not consistent with a fall. The victim had injuries to the head and hands that police said were consistent with blows struck with the silver-colored claw hammer that was found on the ground nearby.

According to the court documents, David DiPaolo told police that Farrar tried to choke him at the base of the cliff. Fearing that he was about to lose consciousness, DiPaolo told police, he hit Farrar in the head with the hammer, according to the criminal complaint.

“DiPaolo insisted that the victim had his hands around DiPaolo’s neck the entire time,” police wrote in the court documents, in which they expressed doubt about the story. The suspect also told police: “I’m sorry this happened. I didn’t want it to happen. I didn’t know it was going to happen.”

Jennifer Jenkins contributed to this report.

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