It sounds like a silencer was used, which explains some of the delay and lack of panic.
Also, if that interviewees account is accurate it sounds like the police were onto to the shooter before he took his shots, so may have been under pressure. Trump was also aware the shooter was 'down' when he stood up and that photo was taken. The conspiracy theories are already rampant across the world, but there's enough info put there that goes someway to explaining the events as they unfolded.
It's certainly a memorable day in global affairs. If Trump has not already won the election then he has now. Reagans approval went up 20 percent after his attempt.