CIUDAD JUAREZ, Mexico – Pointing their rifles, Mexican security forces chased away U.S. authorities investigating the shooting of a 15-year-old Mexican by a U.S. Border Patrol agent on the banks of the Rio Grande, the FBI and witnesses told The Associated Press on Wednesday.

The killing of the Mexican by U.S. authorities — the second in less than two weeks — has exposed the distrust between the two countries that lies just below the surface, and has enraged Mexicans who see the death of the boy on Mexican soil as an act of murder.

Shortly after the boy was shot, Mexican soldiers arrived at the scene and pointed their guns at the Border Patrol agents across the riverbank while bystanders screamed insults and hurled rocks and firecrackers, said FBI spokeswoman Andrea Simmons. She said the agents were forced to withdraw.

“It pretty quickly got very intense over on the Mexican side,” she said, adding that FBI agents showed up later and resumed the investigation, even as Mexican authorities pointed guns at them from across the river.

A relative of the dead boy who had been playing with him told the AP that the Mexicans — whom he described as federal police, not soldiers — pointed their guns only when the Americans waded into the mud in an apparent attempt to cross into Mexico.

The Mexican authorities accused the Americans of trying to recover evidence from Mexican soil and threatened to kill them if they crossed the border, prompting both sides to draw their guns, said the 16-year-old boy who asked not to be further identified for fear of reprisal.

The confrontation occurred Monday night over the body of Sergio Adrian Hernandez Huereka, who died of his wounds beside the column of a railroad bridge connecting Ciudad Juarez, Mexico, and El Paso, Texas. American authorities said he was shot by a Border Patrol agent who was detaining an illegal immigrant trying to enter the U.S. when others started throwing rocks at the agents and refused to stop.

Each government has made veiled accusations suggesting misconduct on the part of the other’s law enforcement agents.

Hernandez was found 20 feet into Mexico. Mexican authorities said a .40 caliber shell casing was found near the body, suggesting the Border Patrol agent might have crossed into Mexico to shoot the boy.