Here are some significant events and policies in the history of U.S. aviation security.
Incident: A United Airlines plane explodes after takeoff in Denver, killing all 44 aboard. Investigators blame Jack Graham for placing a bomb in his mother's luggage.
Incident: Antulio Ramirez Ortiz hijacks a National Airlines flight to Cuba after it takes off in Florida. It is the first aerial hijacking of a U.S. passenger plane.
Policy: The U.S. government begins placing armed guards on commercial planes when requested by airlines or the FBI.
Incidents: Numerous airliners are hijacked to Cuba. Two Palestinian terrorists carry out the first hijacking of a U.S. aircraft outside the Western Hemisphere when they divert TWA Flight 84 0 to Damascus, Syria, after takeoff from Rome.
Policy: The Federal Aviation Administration develops a hijacker psychological profile to be used along with metal detectors.
Incidents: Arab terrorists hijack four airliners, including Pan American World Airways and TWA jets, and blow them up on the ground in the Mideast after releasing all aboard.
Policy: The Customs Air Security Officers Program ("Sky Marshals") is created.
Incidents: At JFK airport, a bomb-sniffing dog finds a device on a TWA plane minutes before it is set to detonate.
Numerous violent hijackings occur in the U.S.
Policies: The FAA creates the Explosives Detection Canine Team Program.
The FAA in December issues an emergency rule requiring all passengers and carry-on baggage to be either screened by metal detectors or searched by hand and requiring airports to station armed guards at boarding checkpoints.
Incident: Two people are killed when a bomb explodes near the Pan Am ticket counter at LAX.
Policy: The 1974 Air Transportation Security Act sanctions the FAA's universal screening rule, which spurs U.S. airports to adopt metal-detection screening portals for passengers and X-ray inspection systems for carry-on bags.
Incident: A bomb concealed in a radio-cassette player destroys Pan Am Flight 103, flying from London to New York, over Lockerbie, Scotland, killing 270 people.
Policy: U.S. carriers at European and Mideast airports begin to require X-rays or searches of all checked baggage and to match passengers and their baggage.
Policy: Airlines begin using a Computer Assisted Passenger Prescreening System to separate passengers who require additional scrutiny, based on behavioral characteristics and a government watch list of known or suspected terrorists.
Incidents: Nineteen Al Qaeda terrorists hijack four U.S. airliners and crash two into New York's World Trade Center towers and one into the Pentagon; the fourth crashes in Pennsylvania.
On a Paris-to-Miami flight on American Airlines, Richard Reid tries to ignite explosives in his shoes; he is overpowered.
Policies: The Transportation Security Administration is formed to oversee security for all modes of travel.
The government orders random inspections of passenger shoes and limits carry-on bags. The government begins banning certain dangerous items, such as scissors, knives and box cutters, from carry-on bags.
Incident: A gunman opens fire at an El Al ticket counter at LAX, killing two people before he is shot to death.
Policies: The government begins requiring passengers to display valid government ID. It restricts access beyond checkpoints.
Incident: British officials foil a plot to blow up aircraft using liquid explosives in carry-on bags, on flights from Britain to the U.S.
Policies: The TSA bans all liquids, gels and aerosols from carry-ons; the ban is later eased.
Incident: A Nigerian is charged with attempting to detonate explosives in his underwear on Northwest Flight 253, en route from Amsterdam to Detroit.
Policies: TSA deploys about 500 Advanced Imaging Technology units, or full-body scanners, to screen passengers. It begins using "enhanced" pat-down procedures at airports for secondary screening and for passengers who decline full-body scanning.
Sources: U.S. Department of Transportation, U.S. Transportation Security Administration, Los Angeles Times, Aviation Safety Network