What the hell happened - and how was it possible? These questions will be at the heart of the FBI investigation in the lead-up up to the attempted assassination of Donald Trump.
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During 25 years with the FBI, I have seen how countless similar rallies were organised and I consider it highly likely that the Secret Service was responsible for security within an enclosed perimeter, while the local police took charge of the wider zone outside.
Responsibility for every post, every zone, every rooftop needs to be clearly assigned and this division of roles should have been clear to everyone.
This is standard practice for the Secret Service. It's carried out at hundreds of events during the run-up to an election but it's also potentially new territory for each local police department.
But why did that sniper ignore Crooks till then? One plausible explanation is that the Secret Service (which is entirely separate from the FBI) assumed the assassin was a police sniper, part of their security team.
That implies serious failures in communication. When it comes to planning for events such as this, I would expect police and Secret Service teams to not only meet and introduce themselves but map out their specific roles in detail. They ought to have been able to recognise each other by sight.
They should also have double-checked at every stage throughout the rally, ensuring they knew who everybody was and what they were doing. That's just basic.
In the coming days and weeks, the agency will reconstruct every detail of the event in Butler, Pennsylvania, using witness statements, CCTV and media video, as well as amateur footage shot on phones by members of the public.