Every anniversary of 9/11 brings with it stories of how our nation's system as it stood on Sept. 10, 2001, was unsuited to stopping al Qaeda. The most popular of these stories focus on criticizing the central role the FBI and the law enforcement community played in fighting terrorism pre-9/11, arguing that because "we treated terrorists like criminals" al Qaeda succeeded in striking America. But the real story of why we failed to prevent 9/11, however, is very different.
To understand how the attacks of 9/11 could have been stopped, one needs to start with the investigation into al Qaeda's bombing of the USS Cole on Oct. 12, 2000. The subsequent investigation by FBI and NCIS Special Agents--through a combination of intelligent detective work and effective (traditional) interrogations--tracked down many of the al Qaeda perpetrators and gained confessions from them.
One of those al Qaeda terrorists, a Yemeni named Fahd al Quso (tasked with videotaping the bombing), confessed to the interrogators that while preparing for the Cole attack he helped transport $36,000 from Yemen to Bangkok, and gave it to a trusted Osama Bin Laden lieutenant, Khallad bin Attash. The investigators followed this lead and found that after Khallad left Bangkok, he traveled to Kuala Lumpur, Malaysia. When the FBI asked the CIA if it knew anything about Khallad being in Malaysia, it said it didn't.
After the attacks of 9/11, the CIA gave the FBI surveillance pictures (that it had since January 2000) of Khallad in Kuala Lumpur at what turned out to be the 9/11 planning meeting. For some reason this information was not shared when requested, nor was the intelligence that Khallad also met Nawaf al Hazmi and Khalid al Mihdhar, two of the eventual 9/11 hijackers, in Bangkok too. After meeting Khallad, the two purchased first-class tickets to Los Angeles--probably using the money Quso brought.