The United States Airline Pilots Association has warned its pilots that terrorists might be conducting 'dry-runs' in their planning to use airplanes in another terror attack.
An internal memo sent by the pilots' union warned the pilots that there have been several cases recently which appear to be probes to test their reaction to an inflight threat, the New York Daily News reports.
This memo cautioned its members to be on the lookout for 'suspicious passengers,' the report added.
The most recent incident occurred on September 2, when a group of Middle Eastern men boarded a flight heading to Orlando from Reagan National Airport.
Later, one of the men ran on the aisle towards the flight deck door, and then abruptly slammed into a bathroom and stayed there for a considerable length of time.
While the suspected man was inside, his fellow passengers roamed inside the cabin, changing seats and opening overhead bins in an apparent attempt to distract the flight crew, the letter added.