CIUDAD JUAREZ, Mexico - A U.S. Border Patrol agent fatally shot a Mexican boy, 15, after a group trying to illegally enter Texas threw rocks at officers near downtown El Paso, U.S. authorities said Tuesday.

Monday evening's shooting, which happened beneath a rail bridge linking the two nations, drew sharp criticism from Mexico. Its government said that "the use of firearms to repel attacks with stones represents disproportionate use of force."

It was the second death of a Mexican at the hands of Border Patrol officers in less than two weeks, and the case threatened to swell into a full-blown international incident.

Arturo Sandoval, a spokesman for the Chihuahua Attorney General's Office, said a spent shell was found near the body - which raises the question of whether the fatal shot was fired inside Mexico. That would violate the rules for Border Patrol agents.

A U.S. official said video showed the Border Patrol agent did not enter Mexico.

The official, who agreed to discuss the matter only if not quoted by name, said the video also showed what seem to be four Mexican law enforcement officers driving to the edge of the dry but muddy bed of the Rio Grande, crossing to the U.S. side, picking up an undetermined object, and returning to Mexico near where the boy's body was.

Mexican law officers, too, are not authorized to cross the border without permission.

According to the FBI, Border Patrol agents were responding to a group of suspected illegal immigrants being smuggled into the United States near the Paso Del Norte bridge.

One suspected illegal immigrant was detained on the levee on the U.S. side, the FBI said. Another Border Patrol agent arrived on the concrete riverbank and detained a second suspect. Other suspects ran back into Mexico and began throwing rocks, it said.

The agent told the rock-throwers to stop, but they continued. He fired his weapon several times, hitting one who later died, the FBI said.

Border Patrol Special Operations Supervisor Ramiro Cordero said the agent was on administrative leave and an investigation was under way.

Chihuahua State officials demanded a full inquiry into the death of the boy, identified as Sergio Adrian Hernandez Huereca. Rosario Hernandez, his sister, said he had been playing with friends and had not planned to cross the border.