Federal prosecutors announced charges Friday against a man they said was upset after last year’s race-tinged Charlottesville clashes and went online to threaten to kill alt-right adherents he believed fueled the violence.

They say Eric M. Radulovic was upset by the death of Heather Heyer, who was killed after a car plowed into a crowd of counterprotesters who’d showed up to oppose the alt-right and white supremacist rally.

Upon learning of follow-up rallies the next week Mr. Radulovic, 31, set about trying to disrupt one in Boston, authorities said, accusing him of going to the 4chan message board and posing as a sympathizer, while threatening to kill some of them.

“I’m going to bring a Remington 700 and start shooting alt-right guys. We need sympathy after that land whale got all the liberals teary eyed, so someone is going to have to make it look like the left is becoming more violent and radicalized,” he said, according to an indictment unsealed Friday in Massachusetts. “It’s a false flag for sure, but I’ll be aiming for the more tanned/dark haired muddied jeans in the crowd so real whites won’t have to worry.”

Prosecutors say that scared people from attending the rally.

Mr. Radulovic was arrested this week in Indiana, where he lives.

The prosecution is the latest fallout of the Charlottesville clash, which pitted far-right protesters against a bigger group of counterprotesters.

James Alex Fields Jr. will stand trial later this year on charges of first degree murder in the death of Heyer.

Some of the organizers of the Charlottesville rally were found guilty last year of misdemeanor charges for failing to disperse the crowd.

Counterprotesters have also been charged with assault after clashing with the white supremacists. One of those, Corey Alexander Long, was in court Friday morning in Charlottesville. A photo from the rally showed Mr. Long, who is black, using an aerosol can as a flame-thrower against men carrying Confederate flags.

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