Mexico has demanded a full inquiry after a Mexican teenager died after a US border patrol agent opened fire from the US side of the border.
The youth, believed to be 14 or 15, was found on the Mexican side under a bridge linking Ciudad Juarez with El Paso.
US authorities said the agent was defending himself and colleagues after they came under attack from people throwing stones and rocks.
The FBI is investigating the incident.
Mexican President Felipe Calderon condemned the shooting and called for the incident "to be thoroughly investigated and those responsible punished", his office said.
"Using firearms to respond to an attack with rocks is a disproportionate use of force, particularly coming from officials that are specially trained," the Mexican foreign ministry said in a statement.
"A worrying increase in the use of excessive force" showed the need for greater co-operation on border security, it added.
The FBI said the agent opened fire during a confrontation with a group of suspected illegal immigrants trying to enter the US.
It said two people were arrested but the others retreated into Mexico, throwing rocks.
"The subjects surrounded the agent and continued to throw rocks at him. The agent then fired his service weapon several times, striking one subject who later died," the FBI said.
The teenager was identified as Sergio Adrian Hernandez.
Photographs showed his body lying on the Mexican side of the Rio Grande.
His mother denied reports he had been trying to cross the border.
"He was not going to cross to El Paso, it is not true, he was here in Mexico. They killed him in Mexico," she said.
A witness told Reuters TV that he saw half a dozen youths being chased back into Mexico after they had briefly crossed the dividing line under the Paso del Norte international bridge.
"They were not carrying anything. They weren't carrying weapons, packages, backpacks, only the clothes they were wearing," said the man who did not want to give his name.
"The agent shot at him twice. He shot at him once, which left him stunned and then he shot him again."
The US Border Patrol says rock attacks on its agents are frequent and capable of causing serious injury.
The issue of illegal migration, and the treatment of Mexicans on the border, is a source of abiding tension between the US and Mexico.
Last week Mexico made similar protests over the death of a Mexican migrant after he was given electric shocks by US agents who were trying to deport him.
Mexico's foreign ministry says its records show so far this year 17 Mexican nationals have been killed or injured by US immigration authorities, up from 12 last year and five in 2008.
There has also been anger in Mexico over a new law in the US state of Arizona, which requires police to question people about their immigration status, if officers suspect they are in the US illegally, and if they have stopped them for a legitimate reason.
But in the US, there is substantial political support for stronger measures to stop the flow of illegal migrants across the Mexican border.
Mexicans make up three-quarters of the estimated 12 million illegal immigrants living in the US.