The faces of the Sept. 11 hijackers and their shameful deeds have been etched into our nation's collective conscience.
However, ABC7 News has learned that the 19 men who used American Airlines and United Airlines planes as missiles may not have been the only terrorists aboard commercial flights on that dark day.
The FBI will not confirm the idea of a fifth plan that was meant to be used during the terror attacks. You will not find information about it in the 9/11 Commission report.
But Lynn Spencer, the author of "Touching History," and Delaware Air National Guard Brigadier General Carol Timmons are talking.
On Sept. 11, Timmons was co-piloting United Airlines flight 23 out of John F. Kennedy International Airport in New York. While in line for takeoff, air traffic controllers closed the airport. She then received a dire message.
"Hijackings, we've had highjackings," Timmons says. "Secure the cockpit. Either land or get back to the terminal."
As Timmons guided her plane back to a gate, she saw the World Trade Center towers on fire. Meanwhile, a chilling call came into the cockpit from a flight attendant.
"The flight attendance says, 'Hey, I've got some very agitated men in sitting in first glass. They are very unhappy. Why aren't we taking off?'" Timmons said.
But once at the gate, the men disappeared in the sea of people evacuating JFK.
"There's the generally held belief that there were other planes out there," Spencer says.
Spencer, a former commercial pilot, wrote "Touching History," a minute-by-minute account of the drama that unfolded on Sept. 11.
She says high ranking FAA officials and airport managers told her that during a search of United flight 23, box cutters and Al-Qaeda documents were found in unclaimed bags.
"It was believed by those who searched the plane and found these items that there were indeed hijackers on that flight.
Spencer says federal officials believe that several additional plans were carrying terrorists that day, but did not become part of the attack after the FAA swiftly shut down American airspace.