Mexican authorities have identified the teenager as Sergio Adrian Hernandez Huereca, 15.

Huereca's mother, Guadalupe Castaneda, said her family is outraged, and they want justice.

"They need to hurry to punish this person," said Castaneda. "My son was only 15-years-old and was not doing anything wrong."

The FBI is the lead agency handling the investigation. The El Paso Police department, Customs and Border Protection Internal Affairs, Immigration and Customs Enforcement and Department of Homeland Security Office of the Inspector General are also investigating the case.

KFOX was the first news station to discuss the investigation with FBI Special Agent Andrea Simmons.

Simmons said around 6:30 p.m. on Monday, Customs and Border Protection Border Patrol agents responded to a group of suspected illegal immigrants, allegedly being smuggled into the U.S. near the Paso Del Norte port of entry.

Simmons said agents detained two of the men, and the others ran into Mexico and began throwing rocks at one of the agents. She said the FBI believes the men then surrounded the agent, he felt threatened and fired his gun.

"With multiple rocks, somebody could be injured," said Simmons. "Somebody could potentially overpower him and get his gun. So you have to take that into consideration."

But Fernando Garcia, executive director of the Border Network for Human Rights called the shooting disturbing, saying an exchange of bullets for rocks seems extreme.

"If the kids were fleeing, they were hiding already, why did they continue shooting at them? We don't know. That's kind of the mystery of this investigation," said Garcia.

He said several people are enraged and scared because this is the second deadly incident involving a Border Patrol agent in two weeks. Mexican immigrant Anastacio Hernandez, 32, died after a Customs and Border Protection officer shocked him with a stun gun at the San Ysidro border crossing that separates San Diego and Tijuana, Mexico. That death has been ruled a homicide.

"I'm not justifying anybody or anything," said Garcia. "I'm just saying that we need to know, what are the protocols? What is the training for the agents to use lethal force? People are afraid because it seems the message is that anyone who tries to cross the border is going to be killed."

But Simmons said she is not familiar with the details of the Border Patrol's use of deadly force policy, and it is not the FBI's concern at this point.

"This is not a civil rights investigation. At this point, it's an assault on a federal officer investigation," said Simmons.

The Border Network for Human Rights has called for a prompt, transparent investigation from the FBI. Garcia said he's concerned about the lack of information released about the incident.

"We hope, as the investigation continues, that we'll be able to provide more information as far as who all was involved and how it all happened," said Simmons. "There may be some other evidence to look at, more witnesses to talk with."

Garcia also questioned why the teenager was shot while on the Mexican side of the border.

Simmons said it would have been very easy for the teenager to cross from one side to the other very quickly.

"At different points, they may go back and forth, but you have to realize that where this happened is very, very close to the border," said Simmons. "We're talking less than 100 yards from the river."

Simmons said the area is a dangerous place where rocks are often thrown at Border Patrol agents.

"There were still individuals throwing firecrackers and rocks when our agents showed up to work this," said Simmons. "So that area has always been a problem."

Simmons said the two men detained by the agents, Oscar Ivan Pineda Ayala and Augustin Alcaraz Reyes, have been charged with illegal entry into the U.S. She said no one has been charged in connection with the alleged assault of the agent. She said the ongoing investigation will determine if any other charges will be filed.

The Border Patrol agent who fired the shots has been placed on paid administrative leave while the investigation is being conducted.

Garcia said the Border Network for Human Rights is gathering people to meet in front of the Santa Fe Bridge at 10 a.m. on Friday to demand a balanced and transparent investigation of this matter.