Mexico says U.S. border agent killed teenager

CIUDAD JUAREZ, Mexico (Reuters) - Mexico demanded an investigation on Tuesday into the apparent shooting death of a teenager by a U.S. border patrol agent, an incident bound to stoke cross-border tensions over illegal immigration.

Witnesses said they saw the unidentified agent shoot at Sergio Hernandez, aged around 14, on Monday under the bridge crossing between El Paso Texas and Ciudad Juarez, a violent city on the frontlines of Mexico's war against drug cartels.

A bystander told Reuters TV that Hernandez was hanging around with friends when the agent shot him.

"We energetically condemn the death of a minor ... near the international border crossing in Ciudad Juarez, when a border patrol official shot at a group of migrants who were apparently throwing rocks," the Mexican foreign ministry said.

"Using firearms to respond to an attack with rocks is a disproportionate use of force, particularly coming from officials that are specially trained," it said in a statement.

But the U.S. Federal Bureau of Investigation, which is leading a multi-agency investigation into the incident, said the agent shot during a confrontation with "suspected illegal aliens" trying to enter the United States.

It said two suspects were arrested and the remainder retreated into Mexico, throwing rocks at the U.S. agents.

"(The) agent ... gave verbal commands to the remaining subjects to stop and retreat. However, 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.

Mexico is a top U.S. trade partner and Washington says the border between the two countries is the busiest in the world.

Hundreds of thousands of Mexicans cross the border illegally into the United States each year to look for work. Many are sent straight home after being caught by U.S. border agents.

U.S.-Mexican relations have been tense since Arizona, a border state, approved a crackdown on illegal immigrants. Mexican President Felipe Calderon criticized the crackdown during a recent trip to the United States.

Mexico's foreign ministry urged security forces on both sides of the border to reconsider use of lethal force. Washington has beefed up border security forces due to spiraling drug violence in northern Mexico.

Lou Patch, a supervisory U.S. border patrol agent in El Paso, said agents may use deadly force when they fear "grievous bodily harm or death to ourselves or another." He said the agent was put on leave.

Andrea Simmons, an FBI spokeswoman in El Paso, said the agent "was on the U.S. side of the border and never left."

A witness who spoke to Reuters TV said he saw half a dozen youths being chased back into Mexico after they had briefly crossed the dividing line under the bridge.

"They were not carrying anything. They weren't carrying weapons, packages, backpacks, only the clothes they were wearing, that's it," the man, who declined to be named, said.

"This one (Hernandez) hid behind the wall. He looked out and that's when (he) was shot," he said. "The agent shot at him twice ... He shot at him once, which left him stunned, and then he shot him again. It seems he was hit in the head."

The dead teenager's mother told reporters at the scene that she was especially angered the U.S. agent had apparently killed her son on Mexican territory.

"I want him punished," she told Mexican television. "He's destroyed my life and left me with nothing."

(Reporting by Julian Cardona in Ciudad Juarez and David Schwartz in Phoenix; and by Mica Rosenberg and Anahi Rama in Mexico City; Editing by Paul Simao)