How the Secret Service protects presidents and what went wrong at the Trump rally [Video]

It typically starts with a meeting with campaign staffers and usually ends without incident — a presidential candidate striding off stage and being whisked to the next stop on the campaign trail.

But the U.S. Secret Service’s advance planning for what outwardly seems like a routine political event is actually an intricate, multilayered process. It typically involves several specialized and senior agents strategizing, assessing and approving a detailed protection mission behind the scenes for days ahead of time, according to five former agents and one current one who have been directly involved in such planning.

For an outdoor campaign event, a comprehensive security plan put into place by the agency generally runs smoothly, said the former agents, some of whom spoke anonymously to discuss security practices.

But last Saturday’s campaign rally that turned deadly in Butler Township, Pennsylvania — with multiple shots fired at former President Donald Trump and into a crowd, killing one man and critically wounding two others — underscores what can …