MOKOLO, Cameroon — As the convoy barreled through a village in northern Cameroon on Monday, a 7-year-old boy darted to the road, excited to see the chain of white S.U.V.s carrying Samantha Power, the first cabinet-level American official to visit the country since 1991.
Distracted by a thundering noise, the boy glanced up at the helicopter providing security from above. Suddenly, he was struck dead — killed by the same convoy that had brought officials to showcase American efforts to help protect West Africa’s women and children.
After hitting the boy, one S.U.V. carrying State Department employees pulled to the side of the road. The rest of the motorcade continued on its way.
“Oh, my God,” Ms. Power said later, after she was informed of the tragedy, according to her spokesman, Kurtis Cooper. “I want to go see his family,” she added, and later she did.
The episode highlighted one of the enduring quandaries of American power. In some remote parts of the world, the only interaction people have with the United States is from afar, whether from the movies produced by Hollywood or the drones overhead that can strike at will.
In this case, Ms. Power, the United States ambassador to the United Nations, had come to West Africa to help raise awareness and win people over. It was planned as part of an effort to convince residents who are terrorized by the Islamist extremist group Boko Haram — but who are also disenchanted by the heavy-handed tactics of their governments — that their paths lie with the American-backed state, not with the militants.
“The United States stands with you,” Ms. Power told local reporters on Monday at the country’s Maroua Airport.
Instead, Ms. Power grimly made her way through a day haunted by the boy’s death. It was a tragedy that often seems on the verge of happening when high-ranking dignitaries drive through the streets in motorcades escorted by local police and security.
The convoys at times hurtle through cities, villages and towns at hair-raising speeds. Villagers, especially children, rush to the side of the road to watch, often running alongside the moving cars as long as they can, shouting and waving.
As Ms. Powers’s convoy left the scene of accident on Monday, an older man ran to the fallen boy, who lay spread-eagled and bleeding on the road. Then the convoy disappeared around a curve and continued on to Mokolo and points beyond where Ms. Power had appointments with women and children victimized by Boko Haram.
The convoy had been driving at more than 40 miles per hour when the vehicle hit the boy. One of the ambulances that was part of Ms. Power’s motorcade was dispatched to transfer the boy to a nearby hospital.
Officials in Ms. Power’s group said later that the convoy continued on its way because the security situation in the area was tense — there had been two Boko Haram attacks nearby just two days before. Ms. Power was not informed of the accident until after she arrived in Mokolo for her first meeting, with a local official.
There was a distinct pall over the rest of the day’s agenda, which was distressing on its own.
In Mokolo, Hulatu Usman, 28, told a harrowing story of running through the bush with her five children with Boko Haram fighters in pursuit. Ms. Usman somehow managed to get all of her children over the border from Nigeria.
At the Minawao camp on Monday afternoon, Ms. Power sat on a mat in a dark room with a 15-year-old girl who told her of being forced to choose between death or marrying a Boko Haram fighter.
As the teenager recounted her tale, she held her baby in her lap, along with a drawing of flowers she had made. All the while, she looked downward, seemingly unable to meet Ms. Power’s eyes.
“The quadruply sad part is that she thinks she made a choice,” Ms. Power, who was visibly upset, told reporters traveling with her. “That was no choice.”
During Ms. Power’s visit, hundreds of children standing at the side of the roads waved and smiled at her.
A short while after the meeting, the motorcade was heading south again. The same Russian-built attack helicopter hovered above, carrying Cameroonian security officials and guarding Ms. Power.
Mr. Cooper said her security officials had balked at returning to the village, but Ms. Power had insisted, saying, “I’m going.”
This time when the convoy arrived in the village, there were no laughing and waving children running on the side of the road. Instead, hundreds of villagers, surrounded by dozens of black-clad Cameroonian soldiers, stood near the road, staring stone-faced at the motorcade.
The vehicles, engines idling, sat on the road for 30 minutes as Ms. Power went in to pay her respects to the boy’s parents.
When she got back in her car, the convoy resumed its journey. For the rest of the way, it continued at a speed of about 25 miles per hour.
Correction: April 22, 2016
Because of an editing error, an article on Tuesday about the death of a Cameroonian boy who was struck by a vehicle in a convoy carrying Samantha Power, the United States ambassador to the United Nations, to meetings with West African victims of the Islamist extremist group Boko Haram referred incorrectly to Mokolo, one of the cities she visited. It is a departmental capital in Cameroon, not the national capital. (That is Yaoundé.)