A Border Patrol agent was charged Monday with first-degree murder in the shooting of an unarmed illegal immigrant at the border in January.

An investigation found that Agent Nicholas Corbett's killing of Francisco Dominguez-Rivera, of Puebla, Mexico, was not legally justified, said Cochise County prosecutor Ed Rheinheimer.

Corbett is also charged with second-degree murder, manslaughter and negligent homicide. A judge will determine which of the charges the evidence supports best, Rheinheimer said.

"We have concluded that the evidence shows that at the time he was shot, Mr. Dominguez-Rivera presented no threat to agent Corbett," Rheinheimer said.

His attorney, Daniel Santander, didn't immediately return a message left Monday afternoon by The Associated Press. The Border Patrol said it would make a statement Tuesday.

The shooting, which drew condemnation from the Mexican government, occurred while Corbett was trying to apprehend Dominguez-Rivera and three others who were trying to enter the country illegally.

In the days after the shooting, the Border Patrol said that a scuffle had led to it and that the agent had "feared for his life."

More than 300 pages of documents later released by prosecutors revealed that Corbett's account didn't match witness testimony or forensic evidence.

Corbett told colleagues he shot at a man who looked like he was going to throw a rock.

But three witnesses who were being apprehended along with Dominguez-Rivera — his two brothers and a sister-in-law — told investigators Corbett fired while pushing Dominguez-Rivera to the ground.

Corbett remains an agent but is not working in the field, said Gus Soto, a patrol spokesman.