Security experts are questioning why the building whose roof served as a shooter's perch for the assailant responsible for Saturday's assassination attempt on former President Trump appeared to not have been secured in advance by law enforcement officials.

"It’s a no-brainer," Kevin Maloy, a former retired special agent with the State Department, told Fox News Digital. "Anybody with a security mindset looked at this and goes, 'What were they thinking? Why was no one there?’"

Thomas Matthew Crooks, 20, fired his weapon at the 45th president from a rooftop about 150 yards away from where Trump was speaking. Crooks pierced the former president’s ear, killed one attendee and critically injured two others before a U.S. Secret Service sniper took him out. Security professionals were baffled why the building in Butler, Pennsylvania, which seemed to pose such an "obvious threat," hadn't been secured. 

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Donald Trump at a campaign rally

Secret Service agents converge to cover Republican presidential candidate former President Donald Trump at a campaign rally, Saturday, July 13, 2024, in Butler, Pa. (AP Photo/Evan Vucci)

"For an elevated shooting platform within 130 or 150 yards of the podium to not be posted or guarded by a policeman or Secret Service person, or at least, access to that platform be restricted, [is a] clear, fundamental failure of the most simple, rudimentary aspects of a security advance," Maloy said. "The security advance is where it all happens. That’s where you do your planning. You lay down your requirements for what is needed to secure the venue."

The Secret Service did not immediately reply to a request for comment from Fox News Digital.

Local authorities were alerted to a suspicious individual by attendees of the rally but could not locate Crooks before he climbed onto the roof and opened fire, Secret Service spokesperson Anthony Guglielmi said on Sunday.

When Secret Service agents first saw the gunman, he was already on the roof firing, according to Guglielmi. At that point, "a Secret Service Counter Sniper neutralized him," Guglielmi said.

But experts said they were perplexed about how Crooks was able to climb on top of the roof in the first place. 

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Donald Trump is surround by U.S. Secret Service agents at a campaign rally,

Republican presidential candidate former President Donald Trump is surround by U.S. Secret Service agents at a campaign rally, Saturday, July 13, 2024, in Butler, Pa. (AP Photo/Evan Vucci)

"Normally you would have had somebody posted on either that roof, because of proximity and everything – putting extra bodies on those buildings would have been a normal protocol," a former White House advance associate told Fox News Digital in an interview. "Or you would have had, what they were probably thinking was, that they had those areas secured by locals because it was far enough out of the bubble."

Normal presidential advance protocol involves Secret Service leadership reviewing and approving security plans, the associate added.

Ken Cuccinelli, the former deputy secretary of Homeland Security, explained that while several law enforcement agency partners are often collaborating to secure campaign rallies and other types of events involving presidential figures, "the Secret Service is always in charge of the protection of their protectees. They don't cede that authority to anyone else, and the law is consistent with that." 

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Donald Trump onstage at a rally

The Secret Service tends to Republican presidential candidate former President Donald Trump onstage at a rally on July 13, 2024, in Butler, Pennsylvania. (Anna Moneymaker/Getty Images)

"But," he said, especially ahead of the GOP convention, the agency is under a lot of "manpower demands."

"There is a major manpower ramp up in presidential campaign years," Cuccinelli said. "They are also occupied preparing and being responsible preparing this national convention."

He added, "I do not understand why that roof wasn't secured. It's too close right to the platform. I don't know why there weren't police officers, not Secret Service agents, but police officers on that roof."

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Gary Seidman, a former Green Beret chief warrant officer, told Fox News Digital that the operation appeared to be a "botch in security of preparation and defense of exterior perimeters."

"The guy that did the actual shooting obtained high ground," Seidman said. "The Secret Service did not do proper assessment and exterior assessment."

He continued, "When some guy with a long gun, meaning not a pistol, climbs up a building and everybody sees it but the Secret Service, that is wrong. And there’s something drastically wrong with that."