The existing security measures flagged more than half of the 19 hijackers in 9/11; however, they were cleared to board the plane because their bags were not found to contain any explosives.
[1] In the months and years following September 11, 2001, security at many airports worldwide were reformed to deter similar terrorist plots.
[3] They also implemented verification tests of the training by projecting images of banned objects on machines to see if workers would be able to identify them.
The TSA also introduced changes on the airplanes themselves, including bulletproof and locked cockpit doors and air marshals which became standard on commercial passenger aircraft.
Security camera footage later showed some hijackers had what appeared to be box cutters clipped to their back pockets.
In addition to standard metal detectors, many U.S. airports now employ full-body scanning machines, in which passengers are screened with millimeter wave technology to check for potential hidden weapons or explosives on their persons.
[citation needed] Initially, early body scanners provoked quite a bit of controversy because the images produced by the machines were deemed graphic and intrusive.
Many considered this an invasion of personal privacy, as TSA screeners were essentially shown an image of each passenger's naked body.
Called a SIDA (Security Identification Display Area) in the U.S., these spaces require special qualifications to enter.
Non-passengers can also no longer meet passengers at their gate inside the terminal, but rather must wait on the other side of the TSA security check.