'TMZ Investigates 9/11: The Fifth Plane' Available to Stream on YouTube

On September 11, 2001, United Flight 23 was grounded when the World Trade Center was hit, but it might have been another plane hijackers planned to weaponize in the terrorist attack -- and, our documentary covering it all is now available on YouTube.

"TMZ Investigates 9/11: The Fifth Plane" -- the result of a 6-month investigation -- takes viewers through the litany of suspicious activities that prompted the FBI to look into whether the plane was also part of the horrific plot that fateful day.

We interviewed 3 flight attendants, the pilot, the United dispatcher, and a member of the 9/11 Commission ... who reveal a story about suspicious passengers who were a bit too eager to get into the air, and uniformed individuals running through the cabin after the plane was evacuated.

The flight's pilot believes these 2 people might have been trying to get weapons or other incriminating evidence off the plane.