Fifth 9/11 plane investigated as terrorist target: ‘There’s a good chance’

The captain of a United Airlines flight scheduled to take off the morning of Sept. 11, 2001, is convinced his plane was intended to be part of the coordinated terrorist attack.

“There is a good chance that somebody was plotting to try to use our airplane as a weapon of mass destruction,” pilot Tom Mannello says in “TMZ Investigates: 9/11: The Fifth Plane,” premiering Monday at 9 p.m. ET on Fox.

TMZ said it spent six months investigating the “suspicious and alarming activities” aboard Flight 23, a Boeing 767 that was due to leave JFK Airport for LA at 9 a.m.

Among the claims: Mannello said he learned two box cutters had been found in the first class seat pockets of the plane parked next to Flight 23 — which had a tail number one digit off.

“If somebody was on the ground cooperating with them, they just simply made a mistake and put the box cutters on the wrong airplane,” Mannello said, claiming it “wouldn’t be the hardest thing in the world” to plant them at the time.

In the hourlong special, flight attendants aboard the plane that day share their suspicions about four people in first class — two men, a child and a person who was dressed in a hijab, with the crew believing it was a man pretending to be a woman — and a man profusely sweating in business class.

“It was odd because it was 8 o’clock in the morning, and airplanes are cold anyway, but it was a cool morning,” flight attendant Sandy Thorngren said of the man’s supposed perspiration.

The flight crew reported struggling to get fruit plates for their first class flyers, who didn’t eat meat, igniting an argument between the passengers and the first class attendant, a woman identified in the documentary as “Deborah.”

“I could hear them say, ‘We do not want to eat, we don’t need food. We want to take off. We don’t need food. We just want to go,’” Thorngren claimed of the first class passengers.

Mannello said he steered the plane toward the runway, unaware of what was unfolding in Manhattan at the time.

He didn’t get cleared to take off though, as air traffic control ordered all flights back to their gates for a mass evacuation.

The attack killed 2,996 people, including 19 hijackers aboard four flights.

American Airlines Flight 11 from Boston to LA struck the North Tower of the World Trade Center at 8:46 a.m., and United Airlines Flight 175, also from Boston to LA, crashed into the South Tower less than 20 minutes later.

American Airlines Flight 77, from Washington DC, struck the Pentagon at 9:37 a.m., while United Flight 93, from Newark to San Francisco, crashed in a Pennsylvania field, likely en route to the Capitol or White House.

Flight 23’s crew believes they could have suffered similar fate.

“I definitely think that Flight 23 from JFK to LAX was the fifth plane,” Thorngren says in the doc. “And that’s what scares, haunts me to this day.”

TMZ reports that 20 minutes after passengers and crew members disembarked the United flight, and the aircraft was locked, people on the ground saw two uniformed individuals running in the passenger cabin.

According to the doc, when authorities came to investigate, they discovered an opened floor hatch, which led from the cabin to the belly of the plane.

Mannello believes the two people may have been searching for the box cutters discovered on the neighboring plane.

TMZ reports the FBI interviewed the flight attendants later that day and took them to a lineup at the Port Authority to see if they could identify the four first class passengers whose behavior drew their suspicion.

“We were escorted to this one room with those double windows where you could see in but not out,” Thorngren says in the doc. “And they asked us if we could identify any of the people that were behind that window.”

No arrests were reported.

The FBI declined to comment to The Post, which also reached out to United Airlines.

After interviewing 1,200 people and reviewing 2.5 million pages of documents, the 9/11 Commission released a nearly 600-page report about the attack in 2004.

Flight 23 was absent from the document.

“I’m deeply concerned that it could be the fifth plane,” former Rep. Carolyn Maloney says in the doc. “I would say that the Intelligence Committee and Congress should look at the intelligence reports that came out from this investigation that took place.”