Gunmen who attacked a pair of U.S. special agents along a rural Mexican roadside Tuesday had a face-to-face exchange with their victims before opening fire, sources familiar with the investigation confirmed to ABC News.
New details of the incident suggest the assailants, who remain at large, knew their targets were U.S. officials, though it's still unclear whether the attack was premeditated.
Investigators believe Immigration and Customs Enforcement agents Jaime Zapata and Victor Avila were returning to Mexico City from a training exercise in Monterrey when their armored SUV with diplomatic plates was spotted going through a toll plaza and became a target.
They were chased by two vehicles, which then forced the agents off the road outside the town of Santa Maria del Rio. At least one of the suspects then approached the Americans' SUV and engaged the ICE agents, who are believed to have been unarmed.
It is unclear what the men said to each other, but U.S. officials, who requested anonymity because the investigation is ongoing, said the agents clearly identified themselves as diplomats. Shortly after that, they were engulfed in a hail of gunfire.
Zapata was killed, and Avila, who was hit twice in the leg, was airlifted to a Mexico City hospital. He has since returned to the U.S. where he's recovering at home.
"This was an intentional ambush against two United States federal agents, which I view as an attack against the United States," said Republican Rep. Michael McCaul of Texas, who sits on the House Homeland Security Committee and has been briefed by ICE on the investigation.
"This tragic event was a game changer," said McCaul. "U.S. law enforcement has operated in Mexico for more than two decades without threat of violence from drug cartels," he said. "The United States will not tolerate acts of violence against its citizens or law enforcement and I believe we must respond forcefully."
President Obama and other top U.S. law enforcement officials have vowed the full force of the federal government to support local Mexican authorities in hunting down Zapata's killers and bringing them to justice.
The FBI announced Wednesday that it was leading a special joint task force between the Department of Homeland Security and Department of Justice to ensure the assailants are caught, with several officers on the ground in Mexico actively supporting the investigation.
"Let me be clear: any act of violence against our ICE personnel -- or any DHS personnel -- is an attack against all those who serve our nation and put their lives at risk for our safety," said Homeland Security Secretary Janet Napolitano. "The full resources of our department are at the disposal of our Mexican partners in this investigation."
While officials have not publicly discussed a possible motive, there is speculation the attack was tied to raging drug-related crime across Mexico that has left 35,000 people dead since December, 2006.
The state of San Luis Potosi, where the attack occurred, has experienced occasional outbursts of violence as cartels have battled for territory and used aggressive tactics to steal cash, weapons and vehicles.