The massive, bronze plaque jutting out from a wall on the east side of the U.S. Capitol says it all.

You may have wondered why everyone on Capitol Hill descended into 19 minutes of mayhem last week when congressional security officials thought an errant aircraft pierced the aviation security bubble surrounding the Capitol.

But the massive plaque near the doorway to the Capitol Rotunda has answers for you. 

"In memory of the passengers and crew of United Airlines Flight 93, whose brave sacrifice on September 11, 2011, not only saved countless lives, but may have saved the U.S. Capitol from destruction," reads the inscription. 

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At 10:03 AM ET on Sept. 11, 2001, Flight 93 crashed near a Shanksville, Pennsylvania, strip mine at 563 miles per hour. U.S. intelligence officials determined years later that Flight 93 was bound for the U.S. Capitol. Before the crash, then-Vice President Dick Cheney had authorized the Air Force to shoot the plane out of the sky. 

Had the heroes aboard Flight 93 not attempted to storm the cockpit, it’s quite possible that the most-hallowed citadel in American democracy may no longer stand. Our world would be a lot different had terrorists destroyed the Capitol on 9/11. 

The Capitol faces lots of security threats all the time. It could be a truck bomb, a sniper, a terrorist posing as a tourist – wearing a suicide vest. Even last year’s riot. 

The Capitol seen at dawn on Nov. 2, 2021.

The Capitol seen at dawn on Nov. 2, 2021. (AP Photo/J. Scott Applewhite, File)

But for more than two decades, one particular threat to the U.S. Capitol stands out above the rest: an airplane.

U.S. Capitol Police made the controversial decision to evacuate the Capitol complex last Wednesday evening. Authorities believed a plane – delivering military parachutists to pre-game ceremonies at nearby Nats Park – posed a threat to the Capitol. 

It turns out it was just a dual-engine De Havilland Twin Otter with blue and yellow U.S. Army markings splashed on the wings and tail. 

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But concerns about air threats are like none other at the Capitol. Warranted or not. 

It’s easy to understand why. 

That stark, bronze plaque attests to echoes of 9/11, which still reverberate across the Capitol’s marble floors and Doric, sandstone columns.

The plane last week circled multiple times inside tenuously restricted airspace near the Capitol as it waited to drop off its passengers above the ballpark. A review by the parachute team – the Army Golden Knights – revealed that the military followed proper protocols and filed appropriate paperwork with the Federal Aviation Administration (FAA). The flight was in contact with the tower at Reagan National Airport, just across the river. However, there appeared to be a question about the flight using the correct radio frequency to communicate with the tower. 

NORAD was not alarmed by the flight. The Pentagon never scrambled the jet fighters it keeps hot 24/7 at Joint Base Andrews to potentially intercept an infringing aircraft.

But the flight perplexed the U.S. Capitol Police. That’s why police commanders ordered an evacuation of America’s seat of government.

"It has to be a split-second decision," said former Senate Sergeant at Arms Drew Willison, who used to be in charge of security for the Senate side of the Capitol. "You can’t start making phone calls at that point. It's too late. You've got to dump the building and get people out and get them safe."

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House Speaker Nancy Pelosi, D-Calif., immediately put out a statement, torching the FAA. 

"The Federal Aviation Administration’s apparent failure to notify Capitol Police of the pre-planned flyover of Nationals Stadium is outrageous and inexcusable. The unnecessary panic caused by this apparent negligence was particularly harmful for members, staff and institutional workers still grappling with the trauma of the attack on their workplace on January 6th," scolded Pelosi.

Lawmakers backed up Pelosi’s admonishment of the FAA. 

The U.S. Army Parachute Team the Golden Knights descends into National Park before a baseball game between the Washington Nationals and the Arizona Diamondbacks Wednesday, April 20, 2022, in Washington.

The U.S. Army Parachute Team the Golden Knights descends into National Park before a baseball game between the Washington Nationals and the Arizona Diamondbacks Wednesday, April 20, 2022, in Washington. (AP Photo/Alex Brandon)

"It was happening at Nationals Park. It is not like it was a secret. So she is quite right to be outraged," said Del. Eleanor Holmes Norton, D-D.C. "And frankly, we are very fortunate that Congress was not in session. 

After a review, the FAA conceded it messed up.

"FAA’s initial review of the circumstances surrounding Wednesday night’s parachute demonstration by the U.S. Army Golden Knights at Nationals Park showed that we did not provide advance notification of this event to the U.S. Capitol Police. We deeply regret that we contributed to a precautionary evacuation of the Capitol complex and apologize for the disruption and fear experienced by those who work there," said the FAA in a statement. 

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Last week’s evacuation was far from the first time that officials ordered a mass evacuation of the Capitol because of a potential air threat. 

The most dramatic example of officials evacuating the Capitol came in June 2004 as President Reagan was about to lie in state in the Rotunda. Dignitaries from around the world descended on the Capitol to pay their respects, including late British Prime Minister Margaret Thatcher and former Soviet leader Mikhail Gorbachev. A military honor guard and horse-drawn caisson was just about to escort Reagan’s casket into the Capitol for a ceremony and public viewing. But security officials believed a plane was headed toward the building. 

Officers raced into the Capitol Rotunda, screaming "Get out! Get out!"

People dashed from the building. Some tripped over one another, sustaining injuries.

It turns out that the Kentucky State Police plane flying then-Gov. Ernie Fletcher to Washington for Reagan’s funeral deviated off course. It was also discovered that Fletcher’s plane had a broken transponder. That prompted the Pentagon to scramble fighters from nearby Joint Base Andrews. U.S. Capitol Police evacuated the building, dignitaries and all.

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Yours truly was told at the time that the military came close to shooting Fletcher’s plane out of the sky.

A private plane wandered off course and creeped too close to the Capitol in May 2005. Capitol Police immediately ordered an evacuation of the Capitol complex.

Military jet fighters scrambled from Andrews fired warning flares at the Cessna, which failed to immediately respond to radio calls. Authorities took the pilot and co-pilot in for questioning once the plane finally landed at Leesburg, Virginia – escorted by a Black Hawk helicopter.

Pelosi was minority leader in 2005. In their haste to evacuate Pelosi, officers on her security detail physically picked up Pelosi with such force that she was lifted out of her leather, slingback shoes. 

Then-Rep. Dave Reichert, R-Wash., found one of Pelosi’s shoes as he ran from the building. Capitol Police officers later located the other. 

In a chapter right out of Cinderella, Reichert showed up a few days later at Pelosi’s weekly press conference and returned the shoe, glass slipper style.

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"Such a gentleman," said Pelosi of Reichert’s chivalry. 

Another air incursion unfolded just weeks later as the House and Senate were voting. Lawmakers dashed down a staircase by the Speaker’s Lobby as they attempted to run as far from the building as they could. 

Yet another stray, civilian aircraft nearly forced an evacuation of the Capitol in April 2009. 

A mailman from Florida flew his gyrocopter from Gettysburg, Pennsylvania, to the West Front Lawn of the Capitol, in May 2015. Postman Doug Hughes literally flew his aircraft "under the radar," eluding detection. Fox was told at the time that the gyrocopter blended in with birds that fly low to the ground.

Secret Service officers search the south grounds of the White House in Washington, Monday, Jan. 26, 2015.

Secret Service officers search the south grounds of the White House in Washington on Jan. 26, 2015. (AP Photo/Susan Walsh)

Those who work on Capitol Hill were ordered to stay away from the building after NORAD detected some sort of bogey on radar early one morning in late November 2019. The Secret Service locked down the White House. The military scrambled fighters from Joint Base Andrews. 

Officials never determined what it was they saw on radar. 

"We don’t know what the hell it was," one senior congressional security official told Fox News at the time. The source speculated that the bogey could have been anything from birds, a drone or even some sort of "weather anomaly."

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But this litany of examples underscores why those who work at the Capitol are rattled by a potential threat from the air. It explains why Capitol security officials take prospective aviation threats so seriously. 

One congressional aide told Fox News she kept watching planes land at Reagan National during last week’s incident, praying that each flight would "turn right," following the Potomac River en route to the airport. 

Everyone is always a little jumpy about this.

And 9/11 and the demise of Flight 93 still resound inside the halls of the U.S. Capitol.