CIUDAD JUAREZ, Mexico - Pointing their rifles, Mexican security forces chased away U.S. officers who were trying to investigate 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 a witness said Wednesday.

The killing of the Mexican - 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.

Mexican news media have been filled with images of the 15-year-old's bloody body and his grieving relatives.

One tabloid ran a large photograph on its cover, with the banner headline "Grindaderas," salty slang that roughly translates as, "Things Americans do."

The Mexican foreign secretary, Patricia Espinosa, said Mexico wasn't taking the Americans' word that the Border Patrol agent was just defending himself from rock-throwers when he opened fire.

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

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

Hernandez was found 20 feet inside Mexico, and an autopsy revealed that the fatal shot was fired at a relatively close range, according to Arturo Sandoval, a spokesman for the Chihuahua State Attorney General's Office.

A U.S. official close to the investigation told the AP that authorities had a video showing that the Border Patrol agent did not cross into Mexico.

The official was not cleared to speak about the video and spoke only on condition of anonymity.

Earlier, migrant Anastasio Hernandez, 42, died after a Customs and Border Protection officer shocked him with a stun gun at the San Ysidro border crossing that separates San Diego from Tijuana, Mexico.

The two deaths have provoked anger in Mexico like no other recent controversy surrounding immigration, including Arizona's new law making it a state crime to be an illegal immigrant and President Obama's decision to send the National Guard to the border.