Watch: Witness says he saw gunman crawl on roof.

One witness has told the BBC's Gary O'Donoghue that he saw a man crawl onto a roof with a rifle, approximately "five to seven minutes" into Trump's remarks.

"We noticed a guy crawling... up the roof of the building beside us, 50 ft away from us," he said.

"We're pointing at the guy crawling up the roof," he said, describing how he and his friends tried to alert law enforcement to the man's presence.