PHOENIX (AP) — A U.S. Border Patrol agent opened fire on a group of people throwing rocks from across the Mexican border, killing a teenage boy and eliciting outrage from the Mexican government over the use of lethal force, authorities said Thursday.

The agents in Nogales, Ariz., had responded to reports of two suspected drugs smugglers near the border at about 11:30 p.m. Wednesday. The agents watched the two abandon a load of narcotics, then run back to Mexico, according to the Border Patrol.

As the agents approached to investigate, people on the Mexican side of the border began throwing rocks at them and ignored orders to stop, the agency said.

One agent opened fire. A Mexican official with direct knowledge of the investigation said Thursday a 16-year-old boy was killed in the shooting. The person spoke on condition of anonymity because they were not yet authorized to discuss details of the case.

The Sonora state attorney general's office in Mexico said in a statement Thursday that Jose Antonio Elena Rodriguez, 16, from Nogales, Sonora, was found dead at the border from gunshot wounds about midnight Wednesday.

However, the office didn't definitively confirm the boy had been shot by the agent, only noting that police received reports of gunshots, then found his body on a sidewalk near the border barrier.

Mexico's Foreign Relations Department issued a statement later saying it "forcefully condemned" the shooting and calling such deaths "a serious bilateral problem."

"The disproportionate use of lethal force during immigration control actions is unacceptable under any circumstances. The repeated nature of this type of cases has drawn a reaction of rejection from Mexican society and all of the country's political forces."

The department said it had asked U.S. authorities for a "exhaustive, transparent and timely investigation" of the shooting.

The Border Patrol declined to comment further and would only say in a statement that one person "appeared to have been" shot by the agent. The FBI was investigating.

Ricardo Alday, a spokesman for the Mexican Embassy in Washington, said in a statement that Mexican authorities will also investigate.

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

In 2010, a 15-year-old boy was shot and killed by a Border Patrol agent firing his weapon 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.

A federal judge 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, the judge noted.

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."

In 2011, a Border Patrol agent shot and killed a man climbing a fence along the Arizona-Mexico. Cochise County sheriff's investigators said at the time there was no indication the 19-year-old assaulted or tried to assault the agent when he was shot three times in the back while climbing a ladder trying to cross the border back into Mexico.

Investigators later found 48 pounds of marijuana in the back of the man's truck. An investigation into the shooting is ongoing.

Another investigation also remains active into a shooting last month by an agent patrolling the Rio Grande.

The Border Patrol said agents were aboard a boat near Laredo, Texas, when a group of people began throwing rocks at them. One of the agents fired shots across the border toward Nuevo Laredo. The agency said it wasn't clear whether anyone had been hit by bullets, but Mexico's Foreign Relations Department issued a statement saying a Mexican citizen had been fatally shot.

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Associated Press writer Mark Stevenson in Mexico City contributed to this report.