Here are some significant events and policies in the history of U.S. aviation security:

1955

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.

1960

A National Airlines plane explodes in midair, killing all 34 aboard. Investigators suspect that a passenger, bent on suicide, had brought a bomb aboard.

1961

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.

1969

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.

1970

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.

1971

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.

1972

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.