After the terrorist attacks
People who operate, service or rent small aircraft should be on the lookout for:
Aircraft with unusual or unauthorized modifications.People loitering for long periods in the vicinity of parked aircraft or in air operations areas.Pilots who appear to be under the control of other persons.People wishing to obtain aircraft without presenting proper credentials.People who present apparently valid credentials but do not have a corresponding level of aviation knowledge.Anything that doesn't look right, such as events or circumstances that don't fit the pattern of lawful, normal activity at your airport.Promptly report information indicating possible criminal activity to your local law enforcement agency. If you see something highly dangerous, such as weapons or explosives being loaded on an aircraft, or if you have other reason to believe a serious crime or some sort of attack is about to occur, immediately call local law enforcement authorities and the nearest FBI office.
Source: Federal Aviation Administration memo