EL PASO, Texas — The FBI and Border Patrol are investigating after the Mexican government said a Mexican citizen was killed when a U.S. agent patrolling the Rio Grande fired his weapon across the border, a Border Patrol spokesman said Thursday.

Border agents were aboard a boat near Laredo when a group of people began throwing rocks at them Monday, Border Patrol spokesman Bill Brooks said. One of the agents fired shots across the border toward Nuevo Laredo.

Brooks said it wasn't immediately clear to the agents whether anyone was hit by the bullets. Mexico's Foreign Relations Department issued a statement Wednesday saying a Mexican citizen had been fatally shot.

FBI spokesman Erick Vasys said his agency is investigating the rock-throwing as an attack on federal officers, while the shooting will be part of an internal review by the Border Patrol.

"We normally and regularly open investigations when a federal officer is assaulted," Vasys said. "Especially if gunfire is involved."

The Mexican foreign relations department's statement protested the use of what it called disproportionate force in immigration control and demanded thorough investigation of the incident. Ricardo Alday, spokesman at the Mexican embassy in Washington, said the Mexican government still is gathering information on the case and also expects to get the results of the investigations by U.S. authorities.

Border agents are generally allowed to use lethal force against rock throwers.

In 2010, a 15-year-old boy was killed by bullets fired by a U.S. Border Patrol agent from El Paso, Texas, into Juarez, Mexico. Some witnesses said people on the Mexican side of the river, including the teen, were throwing rocks at the agent as he tried to arrest an illegal immigrant crossing the Rio Grande.

Senior U.S. District Judge David Briones in El Paso last year dismissed a lawsuit by the family of the boy because the teen was on the Mexican side of the Rio Grande when he was shot. U.S. law gives the government immunity when such claims arise in a foreign country, Briones noted, and the "harm that the Plaintiffs allege ... was felt in Mexico."

A U.S. Department of Justice investigation, which included interviews with more than 25 civilian and law-enforcement witnesses, determined no federal civil rights charges could be pursued because "accident, mistake, misperception, negligence and bad judgment were not sufficient to establish a federal criminal civil rights violation."

"This review took into account evidence indicating that the agent's actions constituted a reasonable use of force or would constitute an act of self-defense in response to the threat created by a group of smugglers hurling rocks at the agent and his detainee," the department said.

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  • FILE - In this April 9, 2009 file photo, a skeletal figure representing the folk saint known in Mexico as "Santa Muerte" or "Death Saint," sits in a vendor's stall at a market in Ciudad Juarez, Mexico. Eight people have been arrested for allegedly killing two boys and a woman in ritual sacrifices by the cult of La Santa Muerte, or Saint Death, according to prosecutors in northern Mexico on Friday March 30, 2012. Jose Larrinaga, spokesman for Sonora state prosecutors, said the first of the three victims was apparently killed in 2009, the second in 2010 and the latest in March 2012. (AP Photo/Rodrigo Abd, File)