But footage and clips of the attack began to proliferate quickly on other platforms, including Twitter and the video publishing site Streamable. Versions of the video reached more than 15,000 accounts on the messaging platform Telegram within half an hour, according to an analysis by Megan Squire , a computer science professor at Elon University in North Carolina.

Ms. Squire, who studies online extremism, said she monitored dozens of “very severely racist, violent channels” that promote white supremacy. On several channels, users debated whether the gunman should be made into a saint, according to screenshots she shared.

“Telegram exists as a forwarding network — that’s the main way the information flows,” she said. “It’s a very efficient mechanism for them.”

The Global Internet Forum to Counter Terrorism , a nonprofit organization formed in 2017 by Facebook, Microsoft, Twitter and YouTube, said in a statement that it was “actively removing perpetrator-created content related to the attack” in an attempt “to prevent its viral spread across our services.” Amazon is also a member of the forum.

The footage that appeared on Twitch was posted from an account created two months ago.

The assailant identified himself in accented English as “Anon” before denying the Holocaust, complaining about feminism and immigration and saying that “the root of all these problems is the Jew.”

He then drove to a synagogue on Halle’s Humboldt Street, an arsenal of weapons visible in his car. He struggled to enter the synagogue. After a woman spoke to him as she passed in the street, he shot her in the back, and then shot her several more times after she collapsed.

Unable to enter the synagogue even after shooting at the door, he drove to a kebab shop. He fired at two men cowering behind a beverage machine and then went outside, where he fired at several pedestrians. Later, he re-entered the shop and shot the body of one victim several times.