United Flight 93, the Boeing 757 that crashed in Pennsylvania when the passengers fought back against the hijackers, was one of the planes he was told could be a threat.
Col. Caine says "not a day goes by that I personally don't remember the incredible courage, leadership and sacrifice that those passengers made. They made our lives that much more peaceful today by taking matters into their own hands, andbeing proactive Americans instead of reactive Americans."
https://www.foxnews.com/us/defending-america-protecting-the-skies-on-sept-11-2001