(CNN) -- At one time, teens Carlos Gonzalez, 16, and Juan Carlos Echeverri, 15, were classmates at Cathedral High School in El Paso, Texas.
Like many who live along the border, both boys had strong ties to El Paso, and especially to neighboring Ciudad Juarez, Mexico. That's where they found themselves on Saturday, where they were gunned down at a car dealership by unknown assailants.
Gonzalez and Echeverri were both U.S. citizens, the Cathedral principal told CNN. A third youth, 16-year-old Cesar Miramontes of Juarez was also killed in the incident.
"Like many kids involved in this border reality of straddling two cultures, he had a lot of stress about what's going on in Juarez," the high school principal, Brother Nick Gonzalez, said of Gonzalez. The two are not related.
2010: Witness to violence: Juarez
The young Gonzalez, the principal said, was "mischievous, but a good kid." His family, which had lived in Juarez, had recently started renting an apartment in El Paso for safety reasons. Juarez is a flashpoint in the drug cartel turf wars that have left thousands dead over the past four years. There were more than 3,000 killings recorded in Juarez alone in 2010.
Moving to the United States, for any child, is a cultural shift, and "I think he struggled with that," the principal said.
Echeverri went to school with Gonzalez at Cathedral their freshman year, before he returned to Juarez to attend school there, the principal said.
The shooting happened at 4:22 p.m. Saturday at a car dealership, where the teens were looking at cars, said Arturo Sandoval, spokesman for the Chihuahua state attorney general's office. Police recovered at least 18 bullet casings at the scene from an AK-47, an automatic weapon commonly used by drug traffickers in the city.
"We believe (Echeverri) and the two others had been friends for a long time. He was here in the city to visit them," Sandoval said.
Virginia Staab, a spokeswoman for the U.S. State Department, said Monday that U.S. authorities are actively looking into the incident and had not yet confirmed the citizenship of any of the victims.
Even though the killings happened in Juarez, they re-open wounds for the students and the community of El Paso, Nick Gonzalez, the principal, said. Seemingly everyone knows someone who has been killed, extorted, kidnapped or robbed, he said.
Just because the violence is on the Mexican side of the border, doesn't make it a Mexican problem, he said.
"It is not a 'them' problem, it is an 'us' problem. We need to find out how to help kids cope," he said. "Just because there is a border doesn't mean that the emotions and pain don't cross over."
Monday's killings marked the first time that a current student at Cathedral High School was killed in the Juarez violence. But alumni have been killed before, and, just before Christmas, the father of a student -- a successful orthopedic surgeon in Juarez -- was kidnapped and killed.
The fatal shootings come on the heels of numerous similar violent incidents in recent months in Juarez. In one 48-hour span last week, for instance, at least 14 people were killed -- including a municipal police officer and a newspaper vendor who police believe was targeted due to her job, according to Juarez Police spokesman Adrian Sanchez.
