A federal appeals court has ruled that the family of a Mexican teenager who was fatally shot by a US agent through a border fence can legally sue the agent and US government for damages.

The Border Patrol agent, Lonnie Swartz, was acquitted in April of second-degree murder in a criminal trial over the 2012 shooting of 16-year-old José Antonio Elena Rodríguez but is facing a retrial in October on lesser charges of voluntary and involuntary manslaughter.

In a separate 2-1 majority decision on Tuesday, the ninth US circuit court of appeals rejected the argument by Swartz’s legal team that the agent was immune from a civil lawsuit brought by Rodríguez’s mother since the boy died on the other side of the border. Swartz shot Rodríguez 10 times through the border fence that separates Arizona from Mexico on 10 October 2012.

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The federal judges wrote that although Swartz’s bullets struck Rodríguez in Mexico, there was a “compelling interest” in regulating the conduct of government agents while standing on US soil.

“Without warning or provocation, Swartz shot [Rodríguez] dead,” the judges stated in their opinion, while adding the agent had violated the fourth amendment of the US constitution that prohibits law enforcement from using “objectively unreasonable” force to seize a person.

Rodríguez “was not committing a crime”, they added.

“He did not otherwise pose a threat to Swartz or anyone else. He was just walking down a street in Mexico.”

Swartz has claimed he acted in self-defense against drug smugglers who were throwing rocks from the Mexican side of the border. Prosecutors have said the agent fired between 14 and 30 bullets across the border, ultimately striking the boy with 10 bullets that mostly hit him in the back.

Swartz is currently on unpaid administrative leave.

The American Civil Liberties Union, which is among the groups representing Rodríguez’s family in the case, hailed Tuesday’s ruling as a “landmark” decision affirming that “unjustified cross-border shootings” are not permissible under the US constitution.

“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.”

