NOGALES, Ariz. — A pair of Mexican drug smugglers in camouflage pants, bundles of marijuana strapped to their backs, scaled a 25 foot-high fence in the middle of the night, slipped quietly into the United States and dashed into the darkness.

U.S. Border Patrol agents and local police gave chase on foot — from bushes to behind homes, then back to the fence.

The conflict escalated. Authorities say they were being pelted with rocks. One agent responded by aiming a gun into Mexico and firing multiple shots at the assailant, killing a 16-year-old boy whose family says was just in the wrong place at the wrong time.

The Oct. 10 shooting has prompted renewed outcry over the Border Patrol’s use-of-force policies and angered human rights activists and Mexican officials who believe the incident has become part of a disturbing trend along the border — gunning down rock-throwers rather than using non-lethal weapons.

The Department of Homeland Security Office of Inspector General has launched a probe of the agency’s policies, the first such broad look at the tactics of an organization with 18,500 agents deployed to the Southwest region alone. The Mexican government has pleaded with the U.S. to change its ways. And the U.N. High Commissioner for Human Rights has questioned the excessive use of force by Border Patrol.

At least 16 people have been killed by agents along the Mexico border since 2010, eight in cases where federal authorities said they were being attacked with rocks, said Vicki Gaubeca, director of the ACLU’s Regional Center for Border Rights in Las Cruces, N.M.

The Border Patrol says sometimes lethal force is necessary: Its agents were assaulted with rocks 249 times in the 2012 fiscal year, causing injuries ranging from minor abrasions to major head contusions.

It is a common occurrence along the border for rocks to be thrown from Mexico at agents in the U.S. by people trying to distract them from making arrests or merely to harass them — particularly in areas that are heavily trafficked by drug smugglers and illegal immigrants.

Still, Gaubeca balks at what she and others deem the unequal “use of force to use a bullet against a rock.”

“There has not been a single death of a Border Patrol agent caused by a rock,” she said. “Why aren’t they doing something to protect their agents, like giving them helmets and shields?”

The Border Patrol has declined to discuss its use of lethal force policy in detail, but notes agents may protect themselves and their colleagues when their lives are threatened, and rocks are considered deadly weapons.

Kent Lundgren, chairman of the National Association of Former Border Patrol Officers, recalled a time in the 1970s when he was hit in the head while patrolling the border near El Paso, Texas.

“It put me on my knees,” Lundgren said. “Had that rock caught me in the temple, it would have been lethal, I have no doubt.”

It is extremely rare for U.S. border authorities to face criminal charges for deaths or injuries to migrants. In April, federal prosecutors said there was insufficient evidence to pursue charges against a Border Patrol agent in the 2010 shooting death of a 15-year-old Mexican in Texas.

In 2008, a case was dismissed against a Border Patrol agent facing a murder charge after two mistrials. Witnesses testified the agent shot a man without provocation but defense attorneys contended the Mexican migrant tried to hit the agent with a rock.

Meanwhile, Mexican families have filed multiple wrongful death lawsuits, and the U.S. government, while admitting no wrongdoing, has paid out hundreds of thousands of dollars. Last year, the family of the illegal immigrant killed by the agent whose murder case was dismissed reached an $850,000 settlement. The agent remains employed by Border Patrol.

Even the Mexican government has asked for a change in policy, to no avail, though Border Patrol points out that Mexico has put up no barriers on its side of the border and does little, if anything, to stop the rock throwers from assaulting agents.

“We have insisted to the United States government by multiple channels and at all levels that it is indispensable they revise and adjust Border Patrol’s standard operating procedures,” Mexico’s Foreign Ministry said in a written statement.

Elsewhere around the world, lethal force is often a last resort in such cases. Israeli police, for instance, typically use rubber bullets, water cannons and tear gas to disperse rock-throwers. Israeli police spokesman Micky Rosenfeld said officers use live fire only as a last resort and first fire warning shots.

“There is no such crowd incident that will occur where the Israeli police will use live fire unless it’s a critical situation where warning shots have to be fired in the air,” Rosenfeld said.

Border Patrol agents since 2002 have been provided weapons that can launch pepper-spray projectiles up to 250 feet away. The agency did not provide statistics on how many times they have been used, but officials are quick to note agents along the U.S.-Mexico border operate in vastly different scenarios than authorities in other countries.

They often patrol wide swaths of desert alone — unlike protest situations elsewhere where authorities gather en masse clad in riot gear.

Experts say there’s little that can be done to stop the violence, given the delicacies of diplomatic relations between the two countries and the fact that no international law specifically covers such instances.

“Ultimately, the politics of the wider U.S.-Mexico relationship are going to play a much bigger role than the law,” said  Kal Raustiala, professor of law and director of the Burkle Center for International Relations at UCLA. “The interests are just too high on both sides to let outrage from Mexico, which is totally understandable, determine the outcome here.”

During a visit to the Border Patrol’s training academy in Artesia, N.M., officials refused comment on all questions about rock-throwing and use of force.

At the sprawling 220-acre desert compound, prospective agents spend at least 59 days at the academy, learning everything from immigration law to off-road driving, defense tactics and marksmanship.

“We’re going to teach them ... the mechanics of the weapon that they’re going to use, the weapons systems, make them good marksmen, put them in scenarios where they have to make that judgment, shoot or not shoot,” said the training academy’s Assistant Chief Patrol Agent James Cox.

In the latest scenario, the two smugglers were attempting to climb the fence back into Mexico, while Border Patrol agents and Nogales Police Department officers ordered them down.

“Don’t worry, they can’t hurt us up here!” one suspect yelled to the other. Then came the rocks.

The police officers took cover, but at least one Border Patrol agent went to the fence and opened fire on Jose Antonio Elena Rodriguez, who was shot seven times, according to Mexican authorities.

The Border Patrol has revealed little information about the case as probes unfold on both sides of the fence that separates Nogales, Ariz., from Nogales, Sonora, literally a stone’s throw from each other.  The FBI is investigating, as is standard with all Border Patrol shootings, and the agency won’t comment “out of respect for the investigative process,” said U.S. Customs and Border Protection spokesman Michael Friel.

The teen’s mother claims her son had nothing to do with drugs or throwing rocks. She says he was just walking past the area a few blocks from home and got caught in the crossfire. None of the training, political maneuvering or diplomatic tip-toeing matters to her. She just wants her boy back. She just wants answers.

“Put yourself in my place,” Araceli Rodriguez told the Nogales International. “A child is what you most love in life. It’s what you get up in the morning for, what you work for. They took away a piece of my heart.”

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Associated Press Writer Josef Federman contributed to this report from Israel.