A video taken just after the shooting captured someone on a nearby roof.

From Tanner Maxwell’s vantage point behind the tent under which Mr. Kirk was speaking at Utah Valley University, he couldn’t see or hear as well as he wanted to, he said in a phone interview.

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He moved to get a better view of the crowd in order to take a video to show his friends. As he pulled his phone out to start filming, he heard a single gunshot. His video, which begins immediately after the shot, captures people dropping to the ground or running for cover.

“It was scary,” Mr. Maxwell said.

Mr. Maxwell rushed home. As he was reviewing his footage later, he noticed a figure running across the roof of a building from which a university spokesperson said the shot that killed Mr. Kirk was fired.

A New York Times analysis of a second video shows what appears to be a person lying on the same roof before the shooting. It is unclear if this person was the shooter, but someone there would have had a clear line of sight to Mr. Kirk. Beau Mason, head of Utah’s Department of Public Safety, said in a press conference that the gunfire that killed Mr. Kirk came from “a longer distance shot, from a roof.”