How did a gunman open fire on a Trump rally?

After a gunman opened fire at former President Donald Trump's rally in Butler, Pennsylvania, security experts said it's challenging to eliminate every threat.

How does a gunman, in plain view of some spectators, manage to open fire on a former president at a campaign rally protected by federal and local law enforcement?

That is the frightening and unanswered question facing the country — and security officials — after Donald Trump said he suffered a graze wound on his right ear while he was speaking in a field in Butler, Pennsylvania, and left the stage with a bloodied head.

Secret Service personnel shot and killed the gunman, identified as Thomas Matthew Crooks of Bethel Park, Pennsylvania, immediately after he opened fire, the FBI said. A spectator was killed and two additional spectators were injured, but it was not immediately clear who shot them.

The assassination attempt, in which Crooks fired multiple shots at the stage from a roof outside the venue’s security perimeter, appeared to be the result of a grave lapse, perhaps the biggest since President Ronald Reagan was shot and wounded outside a Washington hotel in 1981. 

Security experts said it was difficult, and perhaps impossible, to eliminate every threat, particularly with long-range weapons and outdoor events.

https://www.nbcnews.com/politics/donald-trump/gunman-open-fire-trump-rally-rcna161746

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