PHOENIX – 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 drug smugglers near the border 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. He spoke on condition of anonymity because he was not yet authorized to discuss details of the case.

The Border Patrol said it has notified the Mexican government, and the FBI has begun an investigation.

“Preliminary information at this time brings forth, once again, serious doubts about the use of lethal force by U.S. Border Patrol agents, something that both the Mexican government and Mexican society strongly deplore and condemn,” Ricardo Alday, a spokesman for the Mexican Embassy in Washington, said in the statement.

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

In 2010, a 15-year-old boy was killed by a Border Patrol agent firing his weapon from El Paso, Texas, into Juarez, Mexico. Witnesses said the teen and others were throwing rocks at the agent as he tried to arrest an illegal immigrant crossing the Rio Grande.

A federal judge dismissed a lawsuit by the boy’s family because he had been on the Mexican side when he was shot. U.S. law gives the government immunity when such claims arise in a foreign country, the judge noted.