WASHINGTON (AP) — The social media posts are of a distinct type. They hint darkly that the CIA or the FBI are behind mass shootings. They traffic in racist, sexist and homophobic tropes. They revel in the prospect of a "white boy summer."

White nationalists and supremacists, on accounts often run by young men, are building thriving, macho communities across social media platforms like Instagram, Telegram and TikTok, evading detection with coded hashtags and innuendo.

Their snarky memes and trendy videos are riling up thousands of followers on divisive issues including abortion, guns, immi...