EL PASO, Texas – The FBI has opened a civil rights investigation after a Border Patrol agent shot and killed a 15-year-old Mexican boy at the boundary with Ciudad Juarez, Mexico, a U.S. official told The Associated Press on Friday.

Mexican prosecutors, meanwhile, are investigating the case as a homicide, raising the possibility that the agent could also face charges in Mexico, although it is unlikely the United States would agree to extradite him.

The Border Patrol agent was trying to arrest illegal immigrants running into the United States on Monday when he fired his weapon from the U.S. side into Mexico, where people were throwing rocks toward him. A bullet killed Sergio Adrian Hernandez Huereka, who died in Mexican territory. It is still unclear whether Hernandez was throwing rocks.

The FBI formally added the civil rights element to its broader investigation of the alleged assault on the Border Patrol agent, said the official who was familiar with investigation, who spoke only on condition of anonymity.

If investigators determine the Border Patrol agent shot Hernandez without justification, he could be found to have violated Hernandez’s civil rights, which is a crime. The fate of the agent could range from being cleared of wrongdoing to being charged with homicide.

Prosecutors in the Mexican border state of Chihuahua have concluded Hernandez’s death was intentional and homicide charges should be filed against the perpetrator, said Alejandro Parientes, the regional state deputy attorney general. However, state authorities have turned over the case to federal prosecutors for further investigation and a final decision. The federal Attorney General’s Office said there would be no public comment while the investigation continues.

Parientes said his investigation concluded the agent fired his weapon from the U.S. side of the border, but because Hernandez died on the Mexican side, Mexican courts have jurisdiction.

“The homicide had its consequences in Mexican territory,” Parientes said. “What I can say is that we have enough evidence to file homicide charges in a Mexican court.”

The FBI is leading the U.S. investigation because the incident involves a federal officer, whose identity has not been released.

Civil rights investigations are not unusual in cases involving Border Patrol dealings with foreign nationals. In Hernandez’s case, civil rights concerns had always been in the background as the investigation proceeded, but the decision was made to formally embark on that path Thursday.

Last week, a Border Patrol agent pleaded guilty in federal court in Arizona to a criminal civil rights charge for assaulting a Mexican citizen in his custody. In that case, the agent unnecessarily kicked and punched the man, according to the FBI. Sentencing is scheduled for August.

Thomas E. Perez, assistant attorney general for the Civil Rights Division at the Justice Department, said at the time of the Arizona case that his agency “will aggressively prosecute any officer who violates the rights of others and abuses the power they are given to perform their critical duties.”

U.S. Attorney General Eric Holder said Thursday that the possibility of excessive force and violation of civil rights “will certainly be a part of what the investigators will look at” in the El Paso case.

The president of the union representing Border Patrol agents said he feels the civil rights investigation into Monday’s shooting is politically motivated.

“At the beginning, the (Justice Department) was not saying this was a civil rights investigation, then they changed their tune yesterday — in my view, for political reasons,” said T.J. Bonner, president of the National Border Patrol Council.

As the week wore on, condemnations from Mexican officials became increasingly severe.

Mexican Interior Secretary Fernando Gomez Mont called U.S. Homeland Security Secretary Janet Napolitano and Mexico issued a formal diplomatic note expressing concern.