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

The shooting, which happened Monday evening under a railroad bridge linking the two nations, drew sharp criticism from Mexico, where the government said yesterday that "the use of firearms to repel attacks with stones represents disproportionate use of force, particularly coming from authorities who have received specialized training."

The victim was identified as Sergio Adrian Hernandez Huereca. His 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 when U.S. and Mexican officials traded suggestions of misconduct.

Arturo Sandoval, a spokesman for the Chihuahua Attorney General's Office, said a spent 40 caliber shell was found near the body - raising the question of whether the fatal shot was fired inside Mexico, although he did not explicitly suggest that. That would violate the rules for Border Patrol agents, who are supposed to stay on the U.S. side of the border.

A U.S. official, meanwhile, said video shows the Border Patrol agent did not enter Mexico.

The official said the video also shows what seem to be four Mexican law-enforcement officers driving to the edge of the dry but muddy bed of the Rio Grande, walking across to the U.S. side, picking up an undetermined object and returning to Mexico near the area where the boy's body was. Like their U.S. counterparts, Mexican law officers 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 U.S. near the Paso Del Norte bridge, across from Ciudad Juarez, about 6:30 p.m. Monday.

One suspected illegal immigrant was detained on the levee on the U.S. side, the FBI said in a statement. Another Border Patrol agent arrived on the concrete bank where the now-dry, 33-feet wide Rio Grande is, and detained a second suspect. Other suspects ran back into Mexico and began throwing rocks, the FBI said.

At least one rock came from behind the agent, who was kneeling beside the suspected illegal immigrant whom he had prone on the ground, FBI spokeswoman Andrea Simmons said.

The agent told the rock throwers to stop and back off, but they continued. The agent fired his weapon several times, hitting one who later died, said the FBI. It is leading the investigation because a federal officer was assaulted. The agent was not injured, Simmons said.