Hernandez was 15 years old. He died on the spot. He was not armed and, when shot, he was on Mexican soil.

There is dispute about some of these facts. Immediately after the shooting, the CBP claimed that Mesa had fired in self-defense against a rock-throwing crowd. Cellphone videos then surfaced that cast doubt on this. The Arizona Republic reported that one video shows Sergio “peeping out from behind a pillar beneath a train trestle. He sticks his head out; Mesa fires; and the boy falls to the ground, dead.”

There’s been no public official accounting of what happened. The CBP did an investigation itself and concluded there were no grounds to charge Mesa with a crime. Mexico charged him with murder and asked the U.S. to extradite him, which the federal government refused to do. Then Sergio’s parents filed a lawsuit in U.S. federal court against the government and CBP officials, including Agent Mesa. A court dismissed the claims against all defendants; but a panel of the Fifth Circuit reinstated the action against Mesa, holding that the family was entitled to a trial of its claim that the killing of their son amounted to a violation of the Due Process Clause of the Fifth Amendment. The full Fifth Circuit then reversed that decision, dismissing the family’s claims.

Before a trial could be held on the merits, the Supreme Court granted review of that judgment, asking the parties to argue whether the Fourth Amendment (which forbids “unreasonable searches or seizures,” including arrests and shootings) applied to the case; whether that rule was so clear at the time of the shooting that any “reasonable officer” would have known it; and whether the family should be able to sue Mesa under a 1971 case called Bivens v. Six Unknown Named Agents of the Federal Bureau of Narcotics. Because there is no general statute permitting civil-rights suits against federal agents, that case is the vehicle for individual lawsuits against federal officials who violate “clearly established” rights. But it is not a favored child of the current court; indeed, as Justice Anthony Kennedy pointed out in argument Tuesday, “Since 1988, this court has not recognized a single [new kind of] Bivens action.”

So Sergio’s family faced an uphill slog, and its progress wasn’t made easier by their lawyer, Robert C. Hilliard, a successful products liability and personal injury lawyer from Corpus Christi, Texas, who seemed out of his depth in the well of the Supreme Court. Representing Agent Mesa was El Paso attorney Randolph Ortega, a specialist in criminal defense. The government—supporting dismissal of the family’s claim—had Edwin Kneedler of the Solicitor General’s office. Kneedler, who is a veteran of well over 100 arguments in front of the court, was at ease; the other two less so.

Watching oral argument is always an exercise in attempted telepathy: what is going on in the fine legal minds sitting on the bench? Seldom has that seemed more true than in late winter of 2017, as the Trump administration begins assembling the legal and logistical machinery necessary for an all-out assault on America’s undocumented population. The court this term has already heard cases examining whether the Department of Homeland Security can hold aliens indefinitely while seeking to deport them, and whether courts can hold federal officials accountable when evidence shows they deliberately subjected detainees to punitive and unconstitutional conditions in detention. Hernandez—baldly put, asks whether the border is a law-free zone.