Here are some significant events and policies in the history of U.S. aviation security:
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, apparently in hopes of cashing in on her life insurance. It is among the first major acts of criminal violence against a U.S. airliner. Graham is later convicted of murder and executed.
A National Airlines plane explodes in midair, killing all 34 aboard. Investigators suspect that a passenger, bent on suicide, had brought a bomb aboard.
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.
The U.S. government begins placing armed guards on commercial planes when requested by airlines or the FBI.
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 840 to Damascus, Syria, after takeoff from Rome.
The Federal Aviation Administration develops a hijacker psychological profile to be used along with metal detectors to screen passengers and their bags. Eastern Air Lines begins using this system, and several airlines follow.
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.
The Customs Air Security Officers Program ("Sky Marshals") is created to place armed officers, dressed as passengers, on aircraft.
Claiming to have a bomb, a man traveling as D.B. Cooper hijacks a flight in Portland, Ore., as it prepares for takeoff. After arriving in Seattle, he collects $200,000 in ransom and frees the passengers. He boards the plane, forces the crew to take off and parachutes away. He is never found.
The FAA reports that screening of passengers had produced 1,500 arrests and the recovery of a horde of weapons.
After a TWA flight takes off from New York's JFK airport, the airline is notified that a bomb is onboard. The plane returns to the airport, where a bomb-sniffing dog finds the device minutes before it is set to detonate.
Numerous violent hijackings occur in the U.S.
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.
Five Palestinian terrorists open fire at a Rome airport and hurl fire bombs into a Pan Am jet, killing 30 people. They commandeer a Lufthansa jet and fly to Athens; a hostage is killed.
Two people are killed and dozens injured when a bomb explodes near the Pan Am ticket counter at LAX.
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.
Lebanese terrorists divert TWA Flight 847 en route from Athens to Rome; one hostage, a U.S. Navy diver, is killed in the 17-day ordeal.
Federal Air Marshals, successors to the Sky Marshals, become a permanent part of the FAA workforce on international flights.
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.
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.
A plot by Muslim terrorists to blow up 11 U.S. jumbo jets over the Pacific in a single day is uncovered in the Philippines.
TWA Flight 800 explodes after takeoff from New York and crashes into the Atlantic, killing all 230 people aboard. Initially believed to be terrorist-related, the crash is later blamed on equipment failure.
The FAA is provided $100 million for more security personnel and equipment.
Airlines begin using a Computer Assisted Passenger Prescreening System to separate passengers who require additional scrutiny ("selectees"), based on behavioral characteristics and a government watch list of known or suspected terrorists.
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. Thousands die in the deadliest terrorist attack on U.S. soil.
On a Paris-to-Miami flight on American Airlines, Richard Reid tries to ignite explosives in his shoes; he is overpowered.
The Aviation and Transportation Security Act gives the federal government direct responsibility for airport screening. 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 to one bag plus one personal item per passenger. It mandates reinforced cockpit doors on U.S. passenger planes.
The government begins banning certain dangerous items, such as scissors, knives and box cutters, from carry-on bags.
A gunman opens fire at an El Al ticket counter at LAX, killing two people before he is shot to death by a security guard.
The government begins requiring passengers to display valid government ID. It restricts access beyond airport checkpoints.
A federal program is created to arm and train pilots to defend against hijackings and other violence.
By year's end, TSA is screening all checked bags for explosives, prohibited items and other dangerous materials at all U.S. airports. Bags are subject to hand searches.
The TSA abandons plans to test a new Computer Assisted Passenger Prescreening System (CAPPS II) for millions of passengers to detect potential terrorists and begins testing a Registered Traveler program, in which frequent travelers who submit to pre-screening and fingerprinting are allowed to use faster security lines.
British officials foil a plot to blow up aircraft using liquid explosives in carry-on bags, on flights from Britain to the U.S.
All liquids, gels and aerosols are banned from carry-ons; the ban is later eased to allow travel-sized toiletries of 3 ounces or less. TSA loosens its ban on certain sharp objects.
TSA announces it will train more than 500 "behavior detection officers" to scan passengers for suspicious behaviors.
TSA launches the Registered Traveler pilot program.
TSA begins requiring passengers to remove laptop computers, video cameras and other electronics for separate screening.
TSA ends the Registered Traveler program, leaving it to private companies. The private effort falters.
TSA bans lithium batteries from checked luggage unless they are installed in electronic equipment.
A Nigerian is charged with attempting to detonate explosives in his underwear on Northwest Flight 253, en route from Amsterdam to Detroit..
Passengers are forbidden to leave their seats an hour before arrival; the rule is later eased.
TSA deploys about 500 Advanced Imaging Technology units, or full-body scanners, to screen passengers for non-metallic and metallic threats. It begins using "enhanced" pat-down procedures at airports for secondary screening and for passengers who decline full-body scanning.
TSA finishes implementing its Secure Flight program, in which airlines submit passengers' names for matching against a watch list of known or suspected terrorists.
U.S. Department of Transportation, U.S. Transportation Security Administration, Los Angeles Times, Aviation Safety Network