One of the first major attacks on a U.S. airliner was carried out by 23-year old Jack Graham in 1955 on United Airlines Flight 629. Graham placed a bomb in his mother's luggage in the hopes of cashing out on a life insurance policy. All 44 passengers died. He was tried and sentenced to death.
In 1972, the FAA began its canine program after several incidents where bombs downed passenger airplanes. In December of that year, the FAA required all passengers to be inspected by armed guards or screened by metal detectors. A 2013 report has questioned the effectiveness of the canine program.
Following the infamous Lockerbie 1988 bombing of Pam Am Flight 103 from Heathrow to JFK, where a bomb concealed in a cassette player downed a plane killing all 259 onboard, X-ray scanning of all luggage was implemented by U.S., European and Middle East carriers.
Following the events of Sept. 11, 2001, the Transportation Security Adminstration (TSA) was established to create uniform policies to better protect U.S. travel. The TSA was moved to the Department of Homeland Security in 2003.
In 2007, the TSA began deploying some units that used Advanced Imaging Technology-- also known as body scanners. In 2010 full body scanners were installed at airports. However, after much controversy, the TSA began retiring the units starting in late 2012.