CHEMNITZ, Germany — The day after far-right demonstrators took over the streets here, Sören Uhle, a city official who oversees municipal marketing and development, began to get strange phone calls from reporters.

The man whose killing had set off the riots, they said, had died while trying to stop asylum seekers from molesting a local woman. And it wasn’t just one local man who had been killed, but two. Could he comment?

These sorts of accusations suddenly seemed to be everywhere. But none were true. They had come, Mr. Uhle and others suspected, from social media — particularly YouTube.

Ray Serrato, a Berlin-based digital researcher, noticed the tide of misinformation when his wife’s uncle showed him a YouTube video that claimed the rioters had been Muslim refugees.