The Americans Civil Liberties Union notched a key victory when a federal appeals court ruled on Tuesday that its client, the mother of a Mexican teenager who was gunned down by an on-duty U.S. Border Patrol agent, may pursue her Fourth Amendment claims against the agent on behalf of her dead son.

The facts alleged in the case, Rodriguez v. Swartz, are grim. On October 10, 2012, shortly before midnight, Agent Lonnie Swartz was patrolling the United States side of the United States-Mexico border. On the other side of the border, a 16-year-old boy named Jose Antonio Elena Rodriguez (J.A.) was peacefully walking down Calle Internacional, a main thoroughfare in Nogales, Mexico. The border sits between Nogales, Mexico, and Nogales, Arizona, with relatives living on both sides of the border and traversing back and forth to visit their family and to shop.

Without warning or provocation, Swartz fired between 14 and 30 bullets across the border at the teenager, hitting him mostly in the back with about 10 bullets. J.A. was not committing a crime, nor did he pose a threat to Swartz or anyone else. In the wake of the killing, J.A.’s mother, Araceli Rodriguez, sued Lonnie Swartz for monetary damages.

“Her complaint alleges no facts that could allow anyone to characterize the shooting as being negligent or justifiable,” Judge Andrew J. Kleinfeld, a George H.W. Bush appointee, wrote for the three-judge panel. “What is pleaded is simple and straightforward murder.”

Swartz sought to dismiss the Rodriguez’s complaint based on qualified immunity. Qualified immunity is a legal doctrine that protects public officials from lawsuits filed by plaintiffs claiming a violation of their rights as long as the public official’s conduct does not violate a “clearly established” constitutional right. The U.S. Supreme Court has said, in Ashcroft v. al-Kidd, that a right is “clearly established” when it is “sufficiently clear that every reasonable official would have understood that what he is doing violates that right.”

Taking all of the facts in J.A.’s mother’s complaint as true—which, due to a procedural quirk, is what a court ruling on a motion to dismiss must do (even if the facts ultimately turn out not to be true)—a three-judge panel of the Ninth Circuit ruled that Swartz had violated J.A.’s Fourth Amendment rights. The panel also rejected Swartz’s argument that when he fired a dozen or more bullets across the border at J.A. without warning or provocation, it was not clearly established that he couldn’t shoot someone on the other side of the border.

“Qualified immunity exists to protect mistaken but reasonable decisions, not purposeful criminal conduct,” Judge Kleinfeld wrote.

Criminal justice reform advocates have observed that police officials often invoke qualified immunity to insulate police officers when they commit acts of police brutality. It is a powerful doctrine that frequently saves law enforcement officers the ignominy of trial through inquiries regarding how much force an officer was entitled to use and whether the use of that force was reasonable.

But that wasn’t the case here, according to the three-judge panel.

“Swartz did not have to determine how much force to use; he was not permitted to use any force whatsoever against someone who was innocently walking down a street in Mexico,” Kleinfeld wrote, after pointing out that the United States has indicted and tried Swartz for murder.

“We are unable to imagine a serious argument that a federal agent might not have known that it was unlawful to shoot people in Mexico for no reason.”

But that’s what Swartz claimed: Even assuming that he shot and killed J.A. for no reason, Swartz said that he still did not violate the Constitution.

Relying on a 1990 case, United States v. Verdugo-Urquidez, in which the U.S. Supreme Court held that the Fourth Amendment bar on unreasonable search and seizures did not apply to the search and seizure of a Mexican citizen’s property in Mexico, Swartz claimed that he did not violate J.A.’s Fourth Amendment rights when he shot and killed the 16-year-old through the border fence.

In Swartz’s estimation, J.A. was a Mexican citizen who was shot and accordingly “seized” for purposes of the Fourth Amendment in Mexico. Therefore, Swartz reasoned, under Verdugo-Urquidez, he had not violated J.A.’s Fourth Amendment rights.

Not so, according to the three-judge panel. The judges ruled in their decision that Verdugo-Urquidez was inapplicable to Swartz’s case. In Verdugo-Urquidez, Mexican authorities arrested a suspected drug cartel leader named Rene Verdugo-Urquidez in Mexico and handed him over to U.S. law enforcement for trial in the United States. After Verdugo-Urquidez had been transported to the United States, Mexican and U.S. federal agents searched his house without a warrant, and seized evidence showing that Verdugo-Urquidez was a drug smuggler. Verdugo-Urquidez challenged the search on Fourth Amendment grounds.

Verdugo-Urquidez dealt with U.S. agents on Mexican soil, and the U.S. Supreme Court ruled in that case that it would be impractical to apply the Fourth Amendment warrant requirement to property owned by a nonresident and located in a foreign country. Such a search and seizure might potentially conflict with Mexican sovereignty. Whereas in Swartz’s case, he was a U.S. agent on U.S. soil, and he acted subject to U.S. law.

“The concerns in Verdugo-Urquidez were also specific to warrants and overseas operations. But this case is not about searches and seizures broadly speaking,” Kleinfeld wrote. “Neither is it about warrants or overseas operations. It is about the unreasonable use of deadly force by a federal agent on American soil.”

“J.A. had a Fourth Amendment right to be free from the objectively unreasonable use of deadly force by an American agent acting on American soil even though Swartz’s bullets hit him in Mexico,” Kleinfeld concluded.

The decision has been lauded by immigrants’ rights advocates.

“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,” said Lee Gelernt, deputy director of the ACLU’s Immigrants’ Rights Project, which is representing J.A.’s family, according to a statement.

“The ruling could not have come at a more important time, when this administration is seeking to further militarize the border.”

It remains to be seen whether Swartz will ask for a rehearing en banc, before the full court.