Before September 11, 2001, airport screening was provided by private companies which were contracted with the airline or airport. In November 2001, the Transportation Security Administration was introduced to handle screening at all US airports. Private companies still operate screening, but they must be all TSA approved. Argenbright Security, a company that provided security for Newark and Washington Dulles, have had problems before in May 2000, because they hired 1,300 untrained security guards including several dozens with criminal records, at Philadelphia International Airport. The company, which was on probation at the time of the attack, had its probation extended to October 2005.
Box-cutter knives were apparently used in the September 11, 2001 attacks, though such knives are not usually considered weapons. The hijackers could have very easily carried these type of knives past airport security, since they fit the qualifications to be permitted on U.S. domestic flights: any knife with a blade up to 4 inches long was permitted. Box cutters qualified as "menacing" weapons under Hazardous Materials guidelines but were also considered "trade tools" by some airlines. The dual status of these blades caused much confusion for screeners. FAA rules placed into effect on September 13 2001 prohibit any type of knife in secured airport areas and planes. From 2006 onwards, all liquids, gels and aerosols must be less than 3 ounces, and must be placed in a clear plastic bag.
Cockpit doors on many aircraft are now strengthened and bullet proof, to prevent unauthorized access. Some aircraft are equipped with CCTV cameras, so the pilots can monitor the cabin activity and pilots also have an option to carry a gun, but the pilots must be trained to use it. More air marshals have been placed onto flights to improve security and bottles of drinks that are brought onto aircraft must be purchased within the airport.
The airport checkpoint screening has been significantly tightened since 2001. Many passengers are patted-down and thoroughly checked with a hand-held metal detector. The security personnel are also better trained to perform this. On September 11, hijackers Khalid al-Mihdhar, Majed Moqed, Nawaf al-Hazmi and Salem al-Hazmi all set off the metal detector alarm. Despite being checked with a hand-held detector, the screener failed to find the items that caused the alarm. They then all boarded the aircraft. The TSA now test their own screeners, by disguising as passengers and taking in weapons and fake bombs. This is done everyday, and screeners who fail to identify the items are retrained.
On September 11, some hijackers lacked proper ID, yet they were able to board. All passengers generally must now have valid identification, issued by the government in order to fly, although the ID is only visually checked for validity and the name and details are not validated. Airports may check the ID of any suspicious passengers, to ensure that the details on the ID match those on printed boarding card. Under exceptional circumstances, an individual may fly without a valid ID. If approved for flying without an ID, the individual will be subject to a pat-down and wanding. TSA does not have the capability to conduct background checks on passengers at checkpoints.
Many security experts and libertarians have criticized new airport security policies. Bruce Schneier believes that the attackers were successful not because of any particular security screening failure, but because using box cutters to hijack planes and then flying them into buildings had simply never been seriously considered before as an attack vector, although training for this kind of attack was occurring at the exact same time that the attack happened, causing confusion and halting any attempts to stop it. A similar attack attempted today would surely meet with more resistance, as passengers are now fully aware of the potential.
Another common criticism is that any terrorist prevented from carrying a knife onto an airplane could easily improvise a weapon by, for example, smashing a glass bottle - or just attack with his or her bare hands.
Evidence of this can be seen in the events of September 11, 2001 itself, as the passengers on the fourth plane resisted the hijackers once their friends and family called in to report what had happened with the previous three planes. The only difference between this plane and the others was public awareness. Before September 11, 2001, curtains were used to partition the first class cabin from the main cabin. Since the hijackers were all in first class, the main cabin was mostly unaware of what was going on until it was too late. Since then, most airlines have eliminated the curtains, as they pose a security risk and had little other purpose than to symbolically divide the cabins.