Former Florida Gov. Jeb Bush expressed disbelief Wednesday after witnessing two incidents of travelers getting angry about security measures at an airport — on the 18th anniversary of the Sept. 11 terror attacks.

“I am flying today and have seen two incidences where irate passengers are challenging basic security measures,” Bush tweeted. “On September 11. Amazing. I appreciate the patience and professionalism of American Airlines personnel.”

Bush – who made a White House run in 2016 – is the younger brother of George W. Bush, who was president during 9/11 when Al Qaeda terrorists killed nearly 3,000 people after hijacking three airplanes and flying them into the World Trade Center complex in New York City and the Pentagon, outside of Washington, D.C. Another plane, United Airlines Flight 93, was also hijacked but crashed in Pennsylvania after passengers thwarted the terrorists’ attempts to steer the plane to Washington.

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Since the 9/11 attacks, security at airports across the nation has remained heightened and a number of additional measures were implemented.

One of the biggest changes post-9/11 was the creation of the Transportation Security Administration, which came into being when Congress passed a law creating the security organization only a few months after the attack. Before 9/11, airport security was generally handled by private security companies.

One of the main tasks of TSA agents is to inspect travelers and their luggage for weapons, explosive devices and any other banned substances before they head to their gate. This is why – unless a person has TSA PreCheck – travelers have to take off jackets, hats, shoes and belts, and empty their pockets before passing through body-scanning devices.

Along with the routine screening measures, some travelers, especially those on watchlists or who fit a certain profile, are selected for additional screening measures by TSA officials.

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The agency also regulates what travelers can carry on planes with items like knives, box cutters, darts, baseball bats and scissors now banned. The TSA further limits the amount of liquid travelers can take through security.

The 9/11 attacks saw changes inside the airplane too, with cockpits being sealed and locked to prevent passengers from entering and an increase of air marshals on flights.

While all the extra security has forced travelers to arrive at airports much earlier than before the attacks – with it being recommended that travelers show up three hours before their flight when traveling international – it has also been credited with preventing a similar type of attack to the one witnessed 18 years ago.