Wow, that's the first time I heard that he was on the roof for twenty minutes.
I heard he climbed up a ladder and was crawling into position to shoot. And that's when attendees saw him and raised an alarm.
As the ABC NEWS clip showed, the gunman was a person of concern 1hr before the shooting, and as reported the secret service snipers saw him on the roof 20 minutes before the shooting, this is reported at 35-45 seconds and the last 20 seconds in the OP attached ABCNEWS clip?
He was stopped at the metal detectors and backed off into the crowd, he was seen later by officers carrying a "range finder" that's used for shooting judging distance, he was seen walking around the building he used
I personally witnessed the Secret Service Snipers for like 20 seconds having their eyes in their scopes, with rifles directed towards the gunman?
The big question is why the SS snipers didn't shoot the threat, prior to him shooting?