For the first time, a U.S. Border Patrol agent has been charged in a cross-border shooting that ended the life of a Mexican teenager.

A federal grand jury issued an indictment against Border Patrol agent Lonnie Swartz on Wednesday for second-degree murder in the 2012 cross-border shooting of 16-year-old José Antonio Elena Rodríguez.

"The Elena Rodriguez Family is grateful to the DOJ (Department of Justice) for this first step in the pursuit of justice, and remain steadfast in their resolve to seek full transparency from the U.S. Border Patrol on behalf of Jose Antonio," said Luis Parra, an attorney representing Elena Rodríguez's mother, according to The Associated Press.

After Swartz was indicted, his attorney Sean Chapman told The Huffington Post that his client is "going to plead not guilty and fight the charges."

The shooting occurred when Border Patrol agents were responding to reports of drug smuggling in the area around the fence that separates the Mexican city of Nogales from Nogales, Arizona. That's when the agents say someone threw rocks at them. In response, the agents opened fire, fatally shooting Elena Rodríguez in the back multiple times. Elena Rodríguez then died four blocks from his home, according to the Tucson Sentinel.

Witnesses say that the teen was trying to walk home when he was hit by the Border Patrol's bullets, and his death sparked an outcry in Mexico.

According to American Civil Liberties Union attorney Lee Gelernt, who represents Elena Rodríguez's mother in a separate civil lawsuit, the indictment is unprecedented.

"We believe it was justified and that it took too long," Gelernt told HuffPost. "As far as we know, this is the first time there's been an indictment in a cross-border shooting, so it's a big deal ... It sends an enormous message to the Border Patrol going forward that they cannot engage in these types of actions."

"But it also has tremendous importance for the family," Gelernt added. "They've been going against this faceless entity in the United States, not knowing if anyone is taking their claims seriously."

Swartz is expected to appear in court for an arraignment on Oct. 9, reports the Arizona Daily Star.