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The footage, shot on the Upper East Side of Manhattan last October, was disturbing and violent — and quickly went viral. A video showed members of a far-right group , the Proud Boys, striking a smaller group of protesters who appeared to be self-described anti-fascists known as Antifa.

News of the fight, which had broken out after a talk by the Proud Boys founder Gavin McInnes at the Metropolitan Republican Club, came amid increasing national awareness of the rise of neo-fascist and nationalist groups — and their capacity for violence. Video provided crucial evidence that led to the conviction of two men, who were sentenced last week to four years in prison for their part in the melee. But long before the video clips were shown to jurors, they gave reporters a way to understand an opaque event in which witnesses had either vanished or gone silent.

I began looking into what had happened on the Upper East Side within a day or two of the fighting, and later teamed up with a colleague, Ali Winston, to examine the sometimes violent or conspiratorial histories of the Proud Boys involved.