Sergio Hernández Guereca, an unarmed Mexican national, was shot and killed in the summer of 2010 by U.S. Border Patrol Agent Jesus Mesa, who was patrolling the border by bicycle.
How we got here
In 2010, Mesa, while standing on the U.S. side of the border, pointed his service weapon at Hernández, who was on the Mexican side of the border, and struck the teen. Hernández died at the scene of the shooting.
Beyond that, there is little agreement about what happened between the two sides. The facts of the case have never been argued in court, so for the purposes of the Supreme Court hearing, both parties will rely on the account of the facts brought by the petitioners -- the Hernández family.
Hernández was playing a game with friends on the border between El Paso, Texas and Juarez, Mexico in which they would run up and touch the U.S. fence and then run back down, according to court documents.
After Mesa arrived on the U.S. side, he caught one of the boys and the other two ran behind a pillar on the Mexican side of the border, according to court documents. Mesa, who remained on U.S. soil, then shot Hernández as he peered out from pillar.
U.S. authorities initially claimed that Hernández was throwing rocks and Mesa had shot him in self-defense. But cellphone video later revealed that Hernández was shot as he peered his head out, according to the petitioner’s brief.
Hernández’s parents sued Mesa in federal court, but the district court dismissed the claim. The case then moved up to the 5th Circuit of Appeals, which also sided with Mesa.
The Hernández family then appealed to the Supreme Court, which agreed to take the case in October of last year.
"We just want to prove our case in court," said Robert Hilliard, lead attorney representing the Hernández family.
Hernández’s "parents want justice," he said.
Mesa was charged by Mexican authorities, but was never extradited to face those charges.
"We are very confident" that the Supreme Court will find that the opinion of the 5th Circuit is in line with the case law,” said Randolph Ortega, Mesa's attorney.
Mesa, who is still with the Border Patrol, had to uproot his family and re-locate from the El Paso area because of death threats, said Ortega.
"It’s been extremely difficult," he said.
Customs and Border Protection (CBP), which oversees Border Patrol, declined to comment on pending litigation.
The issue before the Supreme Court
The case brings into question the constitutional rights of non-citizens, which could potentially impact other legislation and expand the scope of U.S law.
"The Fourth Amendment protects non-citizens against the arbitrary use of deadly force at the border, at least in the context of a close range, cross border shooting in a confined area patrolled by federal agents," argue attorneys in a brief for the petitioners.
He "was an unarmed civilian and a "member of an intertwined, binational community," said the brief.
But the U.S. government, which is supporting Mesa, said in its brief that U.S. legal protections should not be expanded to non-citizens in this case. “An injury inflicted by the United States on a foreign citizen in another country’s sovereign territory is, by definition, an incident with international implications,” the brief said.
The Supreme Court will also weigh whether Mesa is entitled to "qualified immunity" -- whether an officer is immune from liability for a violation of constitutional rights .
And the justices will also determine whether Hernández’s parents have standing to bring forth the claim in the first place.
Attorneys for Hernández's family, Mesa and the federal government brought their cases before the court on Tuesday.
Hilliard argued before the justices that this "tragic case is one of the most simplest extraterritorial cases this Court will ever have in front of it for five reasons."
He said the reasons are: the conduct of the police officer happened inside the U.S.; it was a civilian domestic police officer; it was a civilian plaintiff, not an enemy combatant; it was one of the most "fundamental rights, the right to life" and lastly; the government of Mexico supports the claim.
The justices repeatedly raised the issue of how a ruling in this case would be used to determine other cases and what those implications would be.
"It seems to me that the principles you're arguing for can't be so narrowly confined. For example, how do you analyze the case of a drone strike in Iraq where the plane in piloted from Nevada?" asked Chief Justice John Roberts.
Hilliard said a ruling in his favor would address the "ongoing problem along the southwest border that has resulted in at least ten cross-border shootings and six Mexican national deaths".
Justice Anthony Kennedy questioned whether this problem would be better solved by Congress and the White House.
"Isn't this an urgent matter of separation of powers for us to respect the ... principle rule that the executive and the legislative have with respect -- respect to foreign affairs?" he asked.
Ortega argued that the Fourth Amendment does not apply in a cross-border shooting of a Mexican civilian on Mexican soil by a United States Federal agent.
"There is no de facto jurisdiction," said Ortega.
Ortega argued that it should be up to the U.S. to decide whether to prosecute Mesa - which the Department of Justice declined to do.
Justice Sonia Sotomayor said that if prosecution were the only option, the family would not be eligible "any damages for the death of their 15-year-old son for their emotional suffering," questioning why there should not be a civil remedy to ensure that Border Patrol are complying with the Constitution.
Deputy United States Solicitor General Edwin Kneedler on behalf of the federal government, argued that plaintiffs here are "seeking to insert the courts into the resolution of a dispute about which the United States and Mexico have a different view of the facts."
While this case is about one incident, Hernández’s parents argue that this is a recurring problem for foreign victims who wish to make claims against the Border Patrol.
In a recent five-year span, border agents shot across the border at least ten times, killing a total of six Mexicans on Mexican soil, according to court documents.
"There is no constitutional constraint when U.S law enforcement stands in the U.S and shoots people. There is no law that governs their conduct," said Hilliard.
After the agency was criticized for transparency and enforcement abuse, former CBP Commissioner Gil Kerlikowske worked to change the culture of the agency, ordering limits on when agents can use their weapons and called for more accountability when civilians are shot.
Kevin McAleenan has been serving as acting commissioner since January 20, 2017.
Violent encounters between CBP officers with both immigrants and American citizens reached a four-year low in 2015, dropping 40 percent from two years earlier.
But those number began to rise again in 2016, with 978 violent encounters recorded in fiscal year 2016, as well as a five-year high of assaults against CBP law enforcement officers.
"CBP uses of force in FY16 were 29% higher than FY15. However, much of the change was due to an increase in the reporting of incidents wherein a less-lethal device was used against multiple subjects. For example, a single utilization of a device such as a Pepperball Launching System (PLS) for area saturation against 15 individuals is counted as 15 total uses of force in one incident," said a spokesperson for CBP.
ABC News' Audrey Taylor contributed to this story.