An analysis of bird's-eye footage of the Butler Farm Show, the venue where Thomas Matthew Crooks shot at former President Trump and killed a bystander, lays bare the apparent tactical shortcomings of the U.S. Secret Service's counter sniper strategy on July 13, according to an expert.

Crooks, 20, killed local firefighter Corey Comperatore and injured 74-year-old James Copenhaver and 57-year-old David Dutch when he fired a volley of rounds toward Trump from a rooftop about 150 yards away at the agricultural fairground in Pennsylvania. 

Former NYPD inspector and Fox News contributor Paul Mauro told "America’s Newsroom" that he spoke with attendees who said the whole event felt "unsecured" in comparison to other rallies. They told him they were not checked with wands or scanned using a metal detector as they entered the venue. 

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The drone footage of the 100-acre site shows the spot where the stage was situated and two red buildings, upon which counter snipers were stationed. 

While Crooks had a full view of them on top of the lower-set gray building across from the stage, Mauro said tree growth and snipers' angle made only part of that roof visible to them.

Mauro spoke with Butler residents who attended past Trump rallies and who said the rally on July 13 felt "very disorganized."

A source familiar with the investigation told Fox News that the sniper who killed Crooks with one bullet had a "one-in-a-million" shot, and was only able to see the top of his head and his gun scope because of the roof's lip. 

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U.S. Secret Service Director Kimberly Cheatle has previously stated that the sloped roof of the building where Crooks perched was a "safety hazard," and that agents were stationed inside the building instead. 

"If you look at the windows that face the stage, they look very narrow and its not entirely clear whether they can even open," Mauro said. "Those windows have been described by federal officials as an observation post. It's hard to see how those low windows, which don't even look like they open and are positioned to ground level would afford a very good observation post for officers inside to look out of."

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Hallways adjoining the building to another beside it provided an access point where Crooks could have clambered onto the roof, Mauro said. An air conditioning unit seen from above may be where he stashed his AR-15 ahead of the rally, sources told Mauro. 

Meanwhile, windows that gave agents a direct vantage point of the roof from a building next door are also visible in the footage.

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"It's almost impossible for them to have missed Tom Crooks clambering onto that roof holding an AR-15," Mauro said. 

Cheatle is appearing before lawmakers on Monday about the assassination attempt. An independent committee has been assigned to review the agency's actions before and during the attack.