How a U.S. Border Patrol argent’s use of lethal force at the U.S-Mexican border implicates constitutional rights and foreign affairs dominated arguments at the U.S. Supreme Court on Tuesday in Hernandez v. Mesa. The lawyer arguing that the agent should be held liable had a rough day in front of the justices.

Both sides agree that while standing on American soil at the border on June 7, 2010, Border Patrol Agent Jesus Mesa fatally shot Sergio Hernandez, a 15-year-old Mexican national standing on the Mexican side. But then the factual accounts diverge.

According to Hernandez’s family, the teenager was playing with his friends near the border opposite El Paso, Texas, where the border runs through the middle of a concrete culvert. There is a fence on the U.S. side of the culvert.

According to Mesa and the federal government, Mesa was detaining one of Hernandez’s companions on the U.S. side of the border, when Hernandez and the other teenagers started throwing rocks at Hernandez. Mesa claims that the rocks posed a danger to his safety. He repeatedly ordered then to stop and back away, but they persisted. Finally Mesa fired in what he claims is self-defense, fatally striking Hernandez.

Hernandez’s family sued, and Mesa filed a motion to dismiss. Under the Federal Rules of Civil Procedure, when considering a motion to dismiss, a federal court must consider the plaintiff’s allegations as true when deciding whether to throw out the lawsuit versus letting it continue. The parties later present evidence to prove their version of the facts if the lawsuit goes forward, but when deciding whether to end the case before it gets started, judges must consider only plaintiff’s version.