Two troubling questions: Why do Mexican youth throw rocks at U.S. Border Patrol agents? And why do Border Patrol agents use lethal force in response?

With the killing of a Mexican youth Wednesday night at the border fence in Nogales, Arizona, three Mexicans have now been shot dead at the border since July.

Mexico’s government voices outrage.

"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," a statement from the Foreign Relations Secretariat says.

A closer look reveals that the cases are a bit more complicated.

In this week’s shooting, Border Patrol agents said they watched Wednesday night at 11:30 p.m. as two Mexicans abandoned a load of narcotics, then ran back to Mexico. 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 U.S. agency said. One agent opened fire.

On Thursday, the Sonora state attorney general's office said in a statement that 16-year-old Jose Antonio Elena Rodríguez of Nogales, Sonora, was found dead on the Mexican side of the border from gunshot wounds.

From a U.S. perspective, these aren’t just teenagers playing pranks. They throw rocks to abet the escape of felons introducing narcotics across international boundaries. And a pattern has been established. You throw rocks at the Border Patrol, you may be shot. Be forewarned. Don't even think about it.

From a Mexican perspective, it’s a disproportionate use of force. The headlines in the Mexican press convey this view. “Border Patrol shoots a Mexican,” Vanguardia said in a headline. “Minor shot in the back by border agent,” El Diario de Sonora said in a headline. Mexican media also suggested that the Border Patrol agents fired across the border, killing the youth in his own country.

Whatever one’s truth, it’s becoming almost a monthly occurrence. In early September, Guillermo Arevalo Pedraza was killed by the Border Patrol near Nuevo Laredo (Spanish language video of incident here). On July 10, Border Patrol agents in Matamoros reportedly shot and killed Juan Pablo Santillan. This story says Santillan was amid a group of rock throwers but that one individual in their group waved a firearm at the Border Patrol agents.

An Associated Press story notes the following:

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.