air marshals have been placed on flights to improve security. Increased security screening On September 11, hijackers Khalid al-Mihdhar, Majed Moqed, and Nawaf al-Hazmi all set off the metal detector. Despite being scanned with a hand-held detector, the hijackers were passed through. Security camera footage later showed some hijackers had what appeared to be box cutters clipped to their back pockets. Box cutters and similar small knives were allowed onboard certain aircraft at the time. Airport checkpoint screening has been significantly tightened since 2001, and security personnel are more thoroughly trained to detect weapons or explosives. 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. 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. Newer body scanners have since been introduced which do not produce an image, but rather alert TSA screeners of areas on the body where an unknown item or substance may be hidden. A TSA security screener then inspects the indicated area(s) manually. Identification checks On September 11, some hijackers lacked proper identification, yet they were allowed to board due to being on domestic aircraft. After 9/11, all passengers 18 years or older in the United States must now have valid government-issued identification in order to fly. Airports may check the ID of any passenger (and staff member) at any time to ensure the details on the ID match those on the printed boarding pass. Only under exceptional circumstances may an individual fly without a valid ID. If approved for flying without an ID, the individual will be subject to extra screening of their person and their carry-on items. TSA does not have the capability to conduct background checks on passengers at checkpoints. Sensitive areas in airports, including airport ramps and operational spaces, are restricted from the general public. Called a SIDA (Security Identification Display Area) in the U.S., these spaces require special qualifications to enter. A European Union regulation demanded airlines make sure that the individual boarding the aircraft is the same individual who checked in his or her luggage; this was implemented by verifying an individual's