In a document that appeared to be posted to the forum 4chan and the messaging platform Discord before the attack, Mr. Gendron explained why he had chosen to stream on Twitch, writing that “it was compatible with livestreaming for free and all people with the internet could watch and record.” (Discord said it was working with law enforcement to investigate.)

Twitch also allows anyone with an account to go live, unlike sites like YouTube, which requires users to verify their account to do so and to have at least 50 subscribers to stream from a mobile device.

“I think that livestreaming this attack gives me some motivation in the way that I know that some people will be cheering for me,” Mr. Gendron wrote.

He also said that he had been inspired by Reddit, far-right sites like The Daily Stormer and the writings of Brenton Tarrant, the Christchurch shooter.