José Antonio Elena Rodríguez, 16, was walking along the border in his hometown of Nogales, Mexico, shortly before midnight in October 2012 when a U.S. Border Patrol agent began firing from the other side of the steel-beam fence. About 10 bullets struck the Mexican teenager in the back, killing him.

The border agent, Lonnie Swartz, said he was firing at a group of teenagers who were hurling rocks at law enforcement as drug smugglers tried crossing into Mexico. But Elena Rodríguez’s mother said the teenager was walking “peacefully” when he was killed. She sued the border agent, accusing him of violating her son’s constitutional rights.

Her lawsuit forced a federal appeals court to delve into a question that has previously left judges divided: Is a Mexican citizen killed on Mexican soil by a U.S. border agent protected by the U.S. Constitution?

The U.S. Court of Appeals for the 9th Circuit held Tuesday that even though the bullets hit Elena Rodríguez in Mexico, the teenager was entitled to constitutional protections against the unreasonable use of deadly force by a U.S. agent standing on the U.S. side of the border.

Swartz argued that his actions are protected by “qualified immunity” as a federal officer doing his job because, among other things, the Constitution offers no protection to noncitizens “with no connections to the United States.”

The court denied qualified immunity for Swartz and said the teenager’s mother can sue him for civil damages.

If the facts presented by the mother are true and the teenager posed no threat to law enforcement, then the border agent’s alleged use of “unreasonable” force violated the Fourth Amendment, wrote Judge Andrew Kleinfeld.

“It is inconceivable that any reasonable officer could have thought that he or she could kill [Elena Rodríguez] for no reason,” Kleinfeld, who was appointed by President George H.W. Bush, wrote in the 72-page opinion.

Kleinfeld was joined on the three-judge panel by Judges Milan Smith and Edward R. Korman. Smith dissented, arguing that the task of deciding whether the mother can sue “lies squarely within the purview of Congress, not of the judiciary.”

In April, a jury acquitted Swartz of second-degree murder, in a case that human rights groups said marked the first time a U.S. Border Patrol agent was prosecuted in a cross-border shooting. But the jury was deadlocked on lesser manslaughter charges, and a mistrial was declared. A retrial in the case is scheduled for October.

In Tuesday’s opinion, Kleinfeld acknowledged that the details in the mother’s lawsuit could turn out to be unsupported. “When all of the facts have been exposed, the shooting may turn out to have been excusable or justified.” he wrote.

The case has striking similarities to a cross-border shooting in June 2010. In that case, a U.S. Border Patrol agent shot and killed 15-year-old Sergio Hernández, a Mexican citizen, in a concrete culvert between El Paso and Juarez, Mexico.

The case, Hernández v. Mesa, made its way to the Supreme Court last year. But the high court declined to rule on whether the family had a right to sue, sending the case back to the lower court.

The U.S. Court of Appeals for the 5th Circuit ruled in March that the teenager was not constitutionally protected on the other side of the border and that the family could therefore not sue for civil damages.

Tuesday’s ruling creates a split between the courts: The 5th Circuit said the Constitution does not extend across the border, but the 9th Circuit now says it does.

For this reason, legal experts say there is a likelihood the Supreme Court will be asked to weigh in on the issue.

“There are many reasons not to extend the Fourth Amendment willy-nilly to actions abroad,” Kleinfeld wrote in Tuesday’s ruling. “But those reasons do not apply to Swartz. He acted on American soil subject to American law.”

He called the border agent’s argument for qualified immunity — that a non-American with no U.S. connection is unprotected by the Constitution — “bizarre.” Swartz could not have known whether Elena Rodríguez was a U.S. or Mexican citizen, Kleinfeld said. For all Swartz knew, the teenager could have been an American with family on both sides of the border.

While the panel cited the Fourth Amendment in its ruling, it also said the Fifth Amendment’s due-process guarantee would apply under a “shocks the conscience” test.

“Swartz’s conduct would fail that test,” Kleinfeld wrote. “We cannot imagine anyone whose conscience would not be shocked by the cold-blooded murder of an innocent person walking down the street in Mexico or Canada by a U.S. Border Patrol agent on the American side of the border.”

The American Civil Liberties Union, which brought the lawsuit on behalf of the family of Elena Rodríguez, praised Tuesday’s decision, calling it a “landmark ruling.”

“The court made clear that the Constitution does not stop at the border and that agents should not have constitutional immunity to fatally shoot Mexican teenagers on the other side of the border fence,” Lee Gelernt, deputy director of the ACLU’s Immigrants’ Rights Project, said in a statement. “The ruling could not have come at a more important time, when this administration is seeking to further militarize the border.”

More from Morning Mix: