A new indictment alleges 58-year-old Mark Adams Prieto -- who's white -- hatched his plan to gun down Black people and other minorities at a hip hop show in Atlanta in mid-May ... and based on the dates the feds mention in their just-unsealed indictment, it lines up with BB's concert dates.

The FBI says it caught wind of Prieto's alleged heinous intentions from a confidential informant and dispatched an undercover agent to get close to him during their probe.

The feds claim Prieto recruited the informant and FBI agent to help him launch his attack, thinking they were in lockstep with his racist views on Blacks, Jews and Muslims. The feds specified in the court docs that Prieto wanted to trigger a "race war prior to the 2024 United States Presidential Election."

Prieto, the feds claim, talked openly about specific details of the alleged planned attack, going so far as to sell a couple of rifles to one of his two "confidantes." They even attached photos.

They allege he told the informants that he chose Atlanta because Georgia used to be a conservative state, but it became "f***ed up" over recent years due to African Americans who have migrated there from other crime-ridden parts of the country.

Prieto is alleged to have said he picked the Bad Bunny concert because of the high concentration of Black people that'd be in attendance. He also allegedly mentioned he'd leave behind confederate flags afterward while yelling "whities out here killing" and "KKK all the way."

The feds say they finally took down Prieto on May 14 -- the day of the Bad Bunny show at State Farm Arena – by pulling over the suspect as he drove through New Mexico. FBI agents say they recovered 7 firearms from Prieto as they arrested him.