EL PASO, Texas – A U.S. Border Patrol agent shot and killed a 15-year-old Mexican boy after a group trying to illegally enter the U.S. threw rocks at the agents at an international bridge near downtown El Paso, authorities said Tuesday.

The shooting, which happened Monday evening, drew sharp criticism from Mexico, where the government said Tuesday that “the use of firearms to repel attacks with stones represents disproportionate use of force, particularly coming from authorities who have received specialized training.”

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, Mexico, around 6:30 p.m. Monday.

One suspected illegal immigrant was detained on the levy on the U.S. side, the FBI said in a statement.

Another Border Patrol agent arrived on the concrete bank where the Rio Grande would be if there was water flowing, 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 he had prone on the ground, said FBI spokeswoman Andrea Simmons.

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, which is leading the investigation because it involved an assault on a federal officer. The agent was not injured.

Border Patrol Special Operations Supervisor Ramiro Cordero said preliminary reports show that Border Patrol agents on bicycle patrol “were assaulted with rocks by an unknown number of people.”

“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.”

Chihuahua state officials released a statement Tuesday demanding a full investigation into the death of the boy, identified as Sergio Adrian Hernandez Huereca.

The boy’s sister, Rosario Hernandez, told Associated Press Television News that her brother was playing with several friends and did not plan to cross the border.

His mother, Maria Guadalupe Guereca, told Milenio TV in Mexico that her son had gone to visit his brother, who handles luggage at a border customs office. While there, he met up with a group of friends and they decided to hang out by the river, she said.

“That was his mistake, to have gone to the river,” she said. “That’s why they killed him.”

She said he ran and hid underneath one of the bridge’s pillars upon hearing gunfire.

“He was a boy, and even then they killed him,” she said. “I ask that they punish them. They left me without anything.”

Sergio Belmonte, spokesman for the Ciudad Juarez mayor, said state justice officials have the body and are performing an autopsy before releasing the body to the family.

Mexico’s Foreign Relations Department said it “energetically condemns” the shooting and demanded “an expeditious and transparent investigation of the facts and, if applicable, punishment of the guilty.”

The department said its records indicated that the number of Mexicans killed or wounded by immigration authorities rose from five in 2008 to 12 in 2009 to 17 so far this year.