On September 11th, 2001, 19 men hijacked 4 planes and killed nearly 3,000 Americans. Did the hijackers have help from a well-established network living in the U.S.? And are some of those who helped the hijackers carry out their attack still on American soil? Fox News Channel tracks down the people and the evidence, getting answers about the mistakes we made and the threats that remain.

Tune in to Fox News Reporting: Secrets of 9/11, Friday at 10p ET hosted by Bill Hemmer. Watch this sneak peek of the investigation:

SLIDESHOW: Behind the Scenes at the 'Secrets of 9/11' Shoot

SNEAK PEEK: Take a Look at Where the 9/11 Hijackers Learned to Fly