RIC FRANCIS/AP

US officials are compiling what one called "growing" evidence that other hijackings may have been planned for September 11. Officials from both the government and the airline industry tell TIME Magazine that a knife-like weapon was found on each of two separate Delta Airlines aircraft later that day, although neither plane took off due to the nationwide grounding after the World Trade Center and Pentagon attacks on hijacked United and American airlines planes.

Government sources would not describe in detail the nature of the weapons found on the Delta flights last week, but one official disclosed that another weapon was discovered on at least on other aircraft, owned by a fourth airline. The government official refused to name that carrier.

Investigators are not yet certain how these weapons came to be on board the aircraft. But they increasingly believe that the weapons may have been prepositioned by accomplices for use by others. As one US official told TIME, "These look like inside jobs."

The new evidence is causing officials to broaden their investigative and security efforts to encompass not only the carry-on bag screening system but the entire aviation security apparatus at US airports. The new evidence raises the worrisome possibility that the hijackers may have had accomplices deep within the 'secure' areas of airports  that may include the shops and restaurants in the terminal behind the metal detectors, or amongst the thousands of people who work in catering, fueling or cleaning aircraft; or anyone who might have access to the airplane before takeoff.

Partly as a consequence, officials tell TIME, the FAA ordered all U.S. airlines on Wednesday to immediately check every single employee on their payrolls against the FBI's watch list of possible suspects. The FAA is also expected to take other actions within days, including possibly revoking contracts with airport security companies or requiring background checks on all airport security personnel.

The senior official confirmed that the air traffic system is on such a hair trigger that military jets have been scrambled more than a dozen times since Sept 11 to investigate other possible hijackings  all of which turned out to be routine mistakes such as transponder problems, the temporary loss of radio contact between an aircraft and the air traffic controllers, or airplanes straying slightly off their intended flight paths.