TMZ Investigates: 9/11: The Fifth Plane

The suspicions caught the interest of the FBI, which interviewed the flight crew and proceeded to take them to a secret lineup to see if they could identify four passengers who were in first class. During that fateful morning, United Flight 23 was called back from the runway after the World Trade Center was hit, and when it returned to the gate, the plane was fully evacuated and locked. However, workers on the ground noticed two uniformed people roaming the cabin 20 minutes after the door was locked, and when authorities boarded the plane to investigate, the hatch that led from the belly of the plane into the passenger cabin was mysteriously open. The pilot states he was told by a United official that boxcutters were discovered in first class seat pockets on the plane parked directly next to Flight 23 -- the tail number was just one digit off -- and the pilot believes someone on the ground mistakenly put the boxcutters on the wrong plane.