(Reuters) - An Arizona man has been indicted on four counts of gun-trafficking crimes after federal prosecutors said an undercover sting operation revealed that he planned to kill Black people in a mass shooting to "incite a race war," according to the indictment.

A grand jury indicted Mark Adams Prieto, 58, on Tuesday on charges of gun trafficking, transferring a gun for use in a hate crime and possession of an unregistered gun.

Prieto was driving east on a New Mexico highway when he was stopped and arrested on May 14 with seven guns in his vehicle, according to the U.S. attorney's office for the Arizona district.

He had earlier told an FBI undercover agent posing as a racist gun enthusiast that he planned to drive to Atlanta, Georgia, for reconnaissance of possible venues to attack, which he wanted to carry out before the U.S. presidential election in November, according to the indictment.

Between January and May, Prieto met repeatedly with the undercover agent and a source secretly helping the FBI at various gun shows around Arizona. Before January, he had talked with the FBI source about "carrying out a mass shooting targeting African Americans, Jews, and Muslims," the indictment said.

Prieto remains in custody and could not be reached for comment, while an attorney representing him did not immediately respond to queries.

The undercover agent, the source and Prieto chose concerts planned for Atlanta on May 14 and May 15 as their targets for the mass shooting, because Prieto expected the concerts would be attended by a large number of African Americans, the indictment said. The indictment did not identify specific events or venues.

In March, Prieto sold to the undercover agent an AR-15 semiautomatic rifle to use in the attack at a Phoenix gun show.

A few weeks later, at another gun show in Phoenix, Prieto told the agent that he wanted to push the attack to a later date, and that he was considering changing the target to a mosque, but that he planned to drive east in May for reconnaissance, the indictment said.

He was driving alone when he was arrested, and while he had seven guns in the vehicle, he only had ammunition for one, the handgun, the indictment said. More guns and ammunition were found in a search of Prieto's home in Prescott, Arizona, including a rifle that was not registered in his name, the indictment said.

He faces a maximum sentence of 15 years in prison if convicted on the most serious charge.

(Reporting by Jonathan Allen in New York; Editing by Rod Nickel)

By Jonathan Allen