TUCSON, Ariz. (AP) — A U.S. Border Patrol agent was fed up with cross-border rock throwers when he deliberately shot at a Mexican teenager, a federal prosecutor said Wednesday as the agent’s second trial began in the 2012 killing.

Lonnie Swartz was acquitted of murder earlier this year in Arizona and now faces manslaughter and involuntary manslaughter charges in the death of 16-year-old Jose Antonio Elena Rodriguez, who was shot in the back multiple times.

It is rare for a Border Patrol agent to be prosecuted on charges involving use of force. Swartz, who has pleaded not guilty, is being retried after the previous jury deadlocked on a manslaughter verdict.

Assistant U.S. Attorney Wallace Kleindienst said in an opening statement that Swartz shot Elena Rodriguez after the teen and others threw rocks at agents who were chasing two drug smugglers on Oct. 10, 2012.

Kleindienst said other agents and police retreated from the rock throwers at the time of the shooting but that Swartz had been involved in previous rock-throwing incidents and had gotten fed up with the tactic used by smugglers to distract agents.

“There is no justification for what he did when took the life Jose Elena Rodriguez,” Kleindienst said.

Defense attorney Sean Chapman said Swartz and other border agents and local police in Nogales, Arizona, were in serious danger. He said Swartz did the right thing by using lethal force because they could have been maimed or killed by the flying rocks.

Chapman said Elena Rodriguez chose to help drug smugglers and endangered the lives of the agents and officers. “This was a smuggling operation. That’s what this case is about,” he said.

In addition to the criminal charges, Swartz is facing a civil rights lawsuit from the American Civil Liberties Union on behalf of the teen’s mother.

The shooting sparked outrage and came at a time when the Border Patrol was increasingly under scrutiny for its use of force, especially in rock-throwing incidents.

The agency has said rocks can be deadly. Chapman says the agency’s training calls for use of deadly force when agents are attacked and believe they’re in serious danger, even if it’s from rocks.

The prosecutor said Swartz didn’t have to shoot the teen or move to another spot and keep firing for a total of 16 shots, 10 of which struck Elena Rodriguez in the back and head.

The trial is expected to last up to a month.