Overview

• On Sept. 11, 2001, 19 al-Qaida hijackers turned four airliners into weapons, killing nearly 3,000 people in New York, at the Pentagon, and in Pennsylvania.
• The United States quickly created the Transportation Security Administration, federalized airport screening, reinforced cockpit doors, and made ID checks and secondary screenings routine.
• After British authorities foiled a 2006 liquid-explosive plot, the TSA imposed the 3.4-ounce liquids rule, required shoe removal, and expanded the presence of federal air marshals.
• By 2008, governments widened use of bomb-sniffing dogs, including screening cargo on passenger flights, adding another visible and invisible layer to airport defenses.
• Since about 2017, airlines and security agencies have rolled out facial-recognition and other biometric systems, with adoption spreading internationally, even as some commentary notes limited rollbacks such as ending shoe removal in certain settings and discussion of revisiting liquids limits.