The disclosure that the video was played shows how Mr. Trump’s anti-media language has influenced his supporters and bled into their own propaganda. Mr. Trump has made attacks on the news media a mainstay of his presidency, and he tweeted a similar — but far less violent video — in 2017. In recent weeks as he has confronted impeachment proceedings, he has ramped up his attacks on the news media, repeatedly calling it the “enemy of the people.”

A person who attended the conference last week took a video of the clip on his phone and had an intermediary send it to a reporter for The Times. Parts of the video were posted on YouTube in 2018 by a user with a history of creating pro-Trump mash-ups.

The organizer of the event said in a statement on Sunday that the clip had been played at the conference, saying it was part of a “meme exhibit.” He denounced the video and said his organization was looking into how it was shown at the event.

“Content was submitted by third parties and was not associated with or endorsed by the conference in any official capacity,” said the organizer, Alex Phillips. “American Priority rejects all political violence and aims to promote a healthy dialogue about the preservation of free speech. This matter is under review.”