A U.S. Border Patrol agent will be retried in what is reportedly the first prosecution of a border agent involving a fatal shooting across the border.

Agent Lonnie Swartz was previously acquitted of second-degree murder in the 2012 death of 16-year-old Mexican youth Jose Antonio Elena Rodriguez, and jurors deadlocked on lesser counts.

On Friday, prosecutors in Tucson, Ariz., said Swartz will now face charges of voluntary and involuntary manslaughter, said Cosme Lopez, a spokesman for the federal court handling the case.

Luis Parra, an attorney for the teen's family, said he was with them in the courtroom when the retrial decision was announced.

The teen, whom authorities say had been throwing rocks, was slain on a street just south of the U.S. border, in Nogales, Mexico.

"I'm relieved and very much appreciate the efforts" of the U.S. attorney's office, Elena Rodriguez’s grandfather said.

Activists had called for the retrial while rallying outside the U.S. District Court building before the hearing.

Lee Gelernt, an attorney for the American Civil Liberties Union in New York City, and deputy director of the group's Immigrants' Rights Project, said the decision to retry the criminal case should not affect the civil action.

The jury in the first trial declared a mistrial last month after it acquitted Swartz of murder and deadlocked on verdicts involving voluntary and involuntary manslaughter.

Elena Rodriguez was killed when Swartz fired 16 shots through a 20-foot fence on an embankment above Calle Internacional, a Nogales street lined with homes and small businesses.

Prosecutors acknowledged during the month-long trial that the teen was lobbing rocks across the border during a drug smuggling attempt but said he did not deserve to die.

Defense attorneys countered that Swartz was justified in using lethal force against rock-throwers and shot from the U.S. side of the border in self-defense.

Court records say the new trial before Judge Raner Collins is set for Oct. 23 and expected to last four weeks.

The Associated Press contributed to this report.