The attack on the World Trade Center on September 11, 2001, remains the largest terrorism event to have occurred on American soil. Compounding the tragedy of that day is the fact information existed that could have stopped the events of 9/11. It's almost impossible to pull something of that size and magnitude off without leaving some kind of a trail, and there were clues about what was about to happen.
9/11 wasn't even the first time the World Trade Center was targeted, and intelligence agencies like the FBI and CIA had already been investigating the possibility of a follow-up attack. Unfortunately, the two agencies' relationship had been strained for decades, and they failed to work together in the months leading up to the attack. Their petty feud ultimately helped lead to the events of 9/11.
The FBI Determined Middle Eastern Terrorists Might Be Training In US Flight Schools But Didn't Tell The CIA
The CIA Refused To Let John P. O'Neill And The FBI Know That Two Al Qaeda Hijackers Were In America
John O'Neill Tried To Share Information With Other Departments
The CIA Learned About A Possible Hijacking But Wouldn't Tell The FBI
O'Neill Was Killed In The World Trade Center On 9/11
This Feud Dates Back To World War II