In a rather frightening example of how easy it can be to blow up a plane, James Bamford, in Pretext for War describes how, in 1995, a terrorist group in the Philippines blew up part of a 747. They used a digital watch with an alarm, some fine wires, a contact lens solution bottle filled with nitro glycerin soaked in cotton, to create a nasty little bomb that was placed under the seat and then detonated 4 hours later after the terrorist had left the plane at an intermediate stop. Note that none of these items would appear suspicious to airport security or show up on an x-ray.
The bomb detonated as planned, killing a Japanese businessman and disabling the plane, which was able to return to the airport with some difficulty. The terrorists were so pleased with their success that they planned several more such attacks. They were thwarted only when their apartment caught fire and a member of the cell was captured. Following interrogation by the Philippine police, it was learned they had also planed [bad pun] to fly an airplane into the Pentagon in a suicide attack. The terrorists claimed the attacks were in protest of American Israeli policies, particularly the savage attack on a Lebanese town in which numerous women and children were killed.
The Philippine police promptly informed the FBI of what they had learned. This information, a preview of the 2001 attack, was either lost or disregarded in one of the intelligence failures that Bamford delineates in a most interesting book. Available from Audible.com unabridged.