Immediately after 9/11, investigators began piecing together the plot. In the flight manifests, they discovered a pattern: A team of five terrorists hijacked every plane that day, except for one. United Airlines Flight 93 had only four hijackers. Investigators believed one person was missing from that group.

Who was the 20th hijacker — if there even was one? And is there a 9/11 hijacker still on the loose? 

Before 9/11, FBI Special Agent John O’Neill kept setting off alarms about the emerging terrorist organization al-Qaeda and their mysterious leader Osama Bin Laden. As a result of his warnings the FBI knew terrorists were planning a big attack and the CIA even had eyes on two of the eventual hijackers prior to the attacks.

So what went wrong? Why didn’t they capture the hijackers? What more could law enforcement have done to stop the attacks? And could they have been prevented? 

After the first two hijackings, the nation’s air traffic controllers quickly and efficiently grounded thousands of planes flying to and over the continental United States. Their quick thinking is said to have potentially saved countless lives.

But what if another plane was meant to be hijacked that day, and never made it off the ground? What if there was a fifth plane on 9/11?

With the Capitol under attack, the President on Air Force One, and the Defense Secretary missing, Vice President Dick Cheney was the most powerful man in the country on the morning of 9/11. From a bunker beneath the White House, he gave fighter pilots flying above the Capitol permission to shoot down hijacked commercial planes.

But did Cheney actually have the authority to issue this order? Did he and President Bush connect before or after Cheney ordered the fighters into battle? And was the U.S. even ready to shoot down any hijacked flights?

Every defense mechanism the U.S. government had on 9/11 failed except for one: the passengers and crew aboard United Airlines Flight 93. They were on the final plane to be hijacked that day and heard reports of the attacks from loved ones on the ground. Then they rallied to stop the terrorist plot.

But where was the plane headed before it crashed? What was the intended target of the Flight 93 hijackers?

After both towers of the World Trade Center were hit on September 11, 2001, first responders coordinated the largest emergency response in New York City’s history. But their heroism at Ground Zero was hampered by institutional bureaucracy and ultimately undone by decisions made decades earlier, when the Twin Towers were built.

So why did these iconic structures and architectural marvels fall so fast? Could more people have been rescued from the towers? And could some of those deaths have been avoided? 

Many of us watched the terrorist attacks on September 11th, 2001 unfold right before our eyes. In the years since, law enforcement, congressional inquiries and the press have successfully uncovered more about what happened that day. But now on the 20th Anniversary of the attacks, many mysteries remain. Hosted by bestselling author and reporter Garrett Graff, Long Shadow answers those lingering questions and explores other enduring mysteries that still surround 9/11, the worst terrorist attack on U.S. soil. Premiering this August, produced by Long Lead and Goat Rodeo.