Updated 6:20 p.m. ET
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 said.
The 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.
A relative of the dead boy who had been playing with him told the Associated Press 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 boy was shot late Monday by a Border Patrol agent who says he was defending himself from rock throwers along the nearly dry Rio Grande that divides Ciudad Juarez, Mexico, from El Paso, Texas.
over the killing of the teen Wednesday and the incident threatened to escalate tensions over migrant issues.
About 30 relatives and friends gathered late Tuesday to mourn Sergio Adrian Hernandez Huereka, whose shooting Monday evening came along the border with Texas. He died on the Mexican side of the river.
"Damn them! Damn them!" sobbed Rosario Hernandez, sister of the dead teenager, at a wake in the family's two-room adobe house on the outskirts of Ciudad Juarez.
Preliminary reports on the incident indicated that U.S. officers on bicycle patrol "were assaulted with rocks by an unknown number of people," Border Patrol Special Operations Supervisor Ramiro Cordero said Tuesday.
"During the assault at least one agent discharged his firearm," he said. "The agent is currently on administrative leave. A thorough, multi-agency investigation is currently ongoing."
The shooting happened beneath a railroad bridge linking the two nations, and late Tuesday night a banner appeared on the bridge that said in English: "U.S. Border Patrol we worry about the violence in Mex and murders and now you. Viva Mexico!"
Less than two weeks ago, Mexican migrant Anastasio Hernandez, 32, 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. The San Diego medical examiner's office ruled that death a homicide.
Mexican President Felipe Calderon said Tuesday that his government "will use all resources available to protect the rights of Mexican migrants."
The government "reiterates its rejection to the disproportionate use of force on the part on U.S. authorities on the border with Mexico," the president added in a statement.
On an unpaved street, gathered around Hernandez's gray metal casket, the teen's family called for justice.
"There is a God, so why would I want vengeance if no one will return him to me. They killed my little boy and the only thing I ask is for the law" to be applied, said the boy's father, Jesus Hernandez.
His mother was less hopeful. "May God forgive them because I know nothing will happen" to them, Maria Guadalupe Huereka said.
Above the casket was a photo of the youth wearing his soccer uniform and his junior high school grade cards, which showed A's and B's.
His mother said he was a good student who never got in trouble. He was the youngest of five children, played on two soccer teams and had just finished junior high school, she said.
The case took a testy turn when U.S. and Mexican officials traded suggestions of misconduct in the incident.
Chihuahua state officials released a statement demanding a full investigation into the death.
The boy's sister, Rosario, told Associated Press Television News that her brother was playing with several friends and did not plan to cross the border.
"They say that they started firing from over there and suddenly hit him in the head," she said.