CIUDAD JUAREZ, Mexico (AP) — 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, 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 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 — who 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 suggesting misconduct on the part of the other's law enforcement agents.

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

That would violate the rules for Border Patrol agents, who are supposed to stay on the U.S. side — and could open the agent to a Mexican homicide prosecution.

A U.S. official close to the investigation told the AP that authorities have a video showing that the Border Patrol agent did not cross into Mexico. In fact, the official said, the video shows what appear to be members of Mexican law enforcement crossing onto the U.S. side, picking something up and returning to Mexico. The official was not cleared to speak about the video and spoke only on condition of anonymity.

Alejandro Pariente, Chihuahua state's regional deputy attorney general, said the U.S. Border Patrol has given him video which he is reviewing. He declined to describe it except to say that it has sped up the investigation.

The two killings 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.

Although many Mexicans were unhappy with both initiatives, popular and official reaction had been subdued, in contrast to street protests seen in previous years when the U.S. has cracked down on the border. Many Mexicans have since given up hope for a quick solution to the immigration problem, while other issues including growing drug violence have taken center stage in relations between the two countries.

That has started to change with the back-to-back deaths of two Mexicans at the border: the teenager killed Monday, and migrant Anastasio Hernandez, 42, who died after a Customs and Border Protection officer shocked him with a stun gun at the San Ysidro border crossing that separates San Diego and Tijuana, Mexico. Anastasio Hernandez, who had lived in the U.S. since he was 14, was buried in San Diego on Wednesday.

Mexican news media were 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."

Mexican President Felipe Calderon pledged to "use all resources available to protect the rights of Mexican migrants," and his foreign secretary, Patricia Espinosa, said Mexico wasn't taking the Americans' word that the Border Patrol agent had been defending himself from rock-throwers when he opened fire.

Chihuahua state Gov. Jose Reyes Baeza blamed the two killings on racism fueled by Arizona's law.

"We believe that this killing, the second in recent days in the border between the two countries, is due to xenophobia and racism, derived from the approval of Arizona's anti-immigration law," Reyes said.

Meanwhile, the Border Patrol released statistics showing that assaults on agents along the border between El Paso, Texas, and Ciudad Juarez, Mexico, were on pace to far exceed totals in the previous four years.

In the past seven months, Border Patrol agents in the El Paso sector have been assaulted 33 times, compared with 39 times in the entire previous year. Twenty-nine of those incidents were rock-throwing, compared to 31 such incidents in all of fiscal 2009.

That's what happened Monday night, when suspected illegal immigrants who ran back to Mexico began throwing rocks at Border Patrol agents detaining other immigrants, Simmons said.

At least one rock came from behind the agent, who was kneeling beside a suspected illegal immigrant whom he was holding prone on the ground, Simmons said. The agent told the rock throwers to stop, then fired his weapon several times, hitting the boy, she said. The FBI is leading the investigation because it involves an assault on a federal officer.

The agent was not injured, Simmons said.

T.J. Bonner, president of the union representing Border Patrol agents, said rock throwing aimed at Border Patrol agents is common and capable of causing serious injury.

"It is a deadly force encounter, one that justifies the use of deadly force," Bonner said.

Mexicans ridiculed that stance.

"Let's say that Anastasio and Sergio Adrian attacked the border agents, one with his fists and the other with rocks," columnist Manuel Jauregui wrote in the newspaper Reforma. "Does that mean that killing them was the only valid option?"