'9/11: The Fifth Plane,' Passengers Argue with Flight Attendants to Take Off

A United 767 scheduled to take off from JFK Airport on September 11, 2001, could have been the 5th plane targeted by hijackers ... this according to the flight attendants and the pilot on that plane. TMZ will present the story of United 23 Monday on FOX at 9 PM ET.

TMZ has conducted a 6-month investigation into United Flight 23 -- scheduled to leave JFK at 9 AM ... bound for Los Angeles. We interviewed 3 flight attendants, the pilot, the United dispatcher, a member of the 9/11 Commission and others about a series of suspicious activities on the plane ... suspicious enough for the FBI to get involved.

Four passengers in first class aroused suspicion shortly after they boarded. The flight attendants say one of the passengers was a man disguised as a woman. The purser [first class flight attendant] tells us those passengers did not eat meat, and there were only 2 fruit plates in the galley. She wanted to get more fruit plates so the passengers could eat, and it triggered a heated argument ... with one of the passengers insisting, "We do not want to eat. We don't need food. We want to take off ... We just want to go."

There were other reasons the flight attendants were suspicious of the 4 passengers in first, along with 2 others in business class.

The plane taxied to the runway and was close to taking off when the World Trade Center was hit and the airport shut down. United 23 went back to the gate and was evacuated -- the plane was empty and locked. Twenty minutes later, people on the ground saw 2 uniformed people running in United 23's passenger cabin. Authorities came a short time later, opened the door and found the hatch -- that led from the belly of the plane to the cabin -- was open.

The pilot believes those 2 people may have been there to remove weapons or other incriminating evidence. We don't know what was removed from Flight 23, but we do know someone put box cutters in the first-class seat pockets in the plane parked next to United 23 ... a plane that was not scheduled to take off at that time. The United 23 pilot believes those boxcutters were intended to be on his flight but someone confused the 2 planes.

The FBI interviewed the flight attendants later that day and even took them to a lineup at the Port Authority to see if they could ID the 4 passengers.