On Monday, a TikTok user with 371 followers, using the screen name "_monix2," posted a video where she read parts of Osama bin Laden's "Letter to America," in which the late terrorist leader said his killings of nearly 3,000 Americans in the Sept. 11, 2001, attacks had been justified by the United States' support of Israel's "occupation" of the Palestinian territories.

By Wednesday night, the letter had become a point of discussion among left-wing creators on the wildly popular video app, with some saying its critiques of American foreign policy had opened their eyes to a history they'd never learned.

But the letter didn't rank among TikTok's top trends. Videos with the #lettertoamerica hashtag had been seen about 2 million times — a relatively low count on a wildly popular app with 150 million accounts in the United States alone.

Then that evening, the journalist Yashar Ali shared a compilation he'd made of the TikTok videos in a post on X, formerly Twitter. That post has been viewed more than 28 million times. By Thursday afternoon, when TikTok announced it had banned the hashtag and dozens of similar variations, TikTok videos tagged #lettertoamerica had gained more than 15 million views.

The letter's spread sparked a deluge of commentary, with some worrying that TikTok's users were being radicalized by a terrorist manifesto, and TikTok's critics arguing it was evidence that the app, owned by the Chinese tech giant ByteDance, had been secretly boosting propaganda to a captive audience of American youth.

But the letter's spread also reflected the bedeviling realities of modern social media, where young people — many of whom were born after 9/11 — share and receive information on fast-paced smartphone apps designed to make videos go viral, regardless of their content.