It was an annual tradition, a 130-mile ride on long stretches of highway from the M Resort Spa Casino in Henderson, Nev., through Searchlight to Nipton, Calif., and back.
But at 9:39 a.m. on Thursday, about 40 miles into the ride, a box truck slammed into the group of about 20 cyclists as they rode on the shoulder of U.S. 95 in Clark County, just north of Searchlight, according to the Nevada Highway Patrol.
Five cyclists were killed and four were injured, including one who was in critical condition, the Highway Patrol said.
The Highway Patrol said investigators did not know why the truck had plowed into the group but said the driver had left the roadway, hit the group from behind and then struck a Subaru hatchback that was accompanying the cyclists and another group of cyclists that was in front of the Subaru.
“Three were dead instantly, as you could tell from their bodies,” Michael Anderson, who had organized the ride and was cycling ahead of his friends when they were hit, said in a phone interview. “I watched two die in front of me. I have been crying all day.”
Trooper Travis Smaka, a Highway Patrol spokesman, said the driver had stayed at the scene of the crash and was cooperating with the investigation.
Investigators do not believe that the driver was impaired by alcohol or drugs, and no charges have been filed as the investigation continues, he said.
The authorities have not released the names of the four men and one woman who were killed or the name of the driver. The speed limit on that portion of the highway is 75 m.p.h.
Mr. Anderson said the cyclists had split into two groups — with stronger riders ahead of the car that was accompanying them and slower riders drafting behind it — when the truck slammed into the group that was trailing the car.
He said he had been in the group ahead of the car and was told by a passing driver that the cyclists behind him had been hit.
“We turned around and went back, and that’s when all hell broke loose — when I saw my friends all over the road,” Mr. Anderson said.
Mr. Anderson, who retired last month after 22 years with the Las Vegas Metropolitan Police Department, said he had responded to crashes before, but nothing like this one.
“I felt helpless that my friends were literally there, shredded,” he said. “I can’t think of a more devastating crash ever here.”
Clay Weeks, who works at Pro Cyclery, a bike shop in Las Vegas, said he knew some of the riders that were in the group, including a mechanic who works at the store who was unharmed but “very emotionally hurt.”
“It’s a road that typically would be safe,” Mr. Weeks said. “The shoulder is plenty wide to ride out there. It’s not a narrow, sketchy road.”
He said news of the crash had traveled quickly through the close-knit cycling community.
“Everyone is just devastated,” he said. “Hopefully this opens eyes for people and will make them more vigilant of cyclists on the road because stuff like this happens way too often in the community.”
Bicycle rides account for only 1 percent of all trips in the United States, but cyclists face a higher risk of crash-related injuries and death than people in cars and trucks, according to the Centers for Disease Control and Prevention.
Last year, the National Highway Traffic Safety Administration reported that more pedestrians and cyclists had been killed in 2018 in the United States than in any year since 1990.
The number of pedestrians killed grew by 3.4 percent in 2018, to 6,283, and the number of cyclists killed rose by 6.3 percent, to 857, even as total traffic deaths decreased, the agency said. On average, about 17 pedestrians and two cyclists were killed each day in crashes.