The "best chance" of preventing the 9/11 terror attacks was directly under US intelligence authorities' noses, but they had no idea just how close they came, a chilling report revealed.
In fact, two of the hijackers were even sharing a house with an FBI informant.
Not only did the FBI fail to notice suspects who were living in plain sight, authorities missed multiple warnings which could have prevented hijackers slamming planes into the World Trade Centre, the Pentagon, as well as the failed attack on the White House which crash-landed in a field in Pennsylvania.
A number of frustrating sequences of events have been unearthed to mark the 10-year anniversary of the public release of the Office of the Inspector General's (OIG) review of the FBI's handling of intelligence information related to the September 11 attacks.
These include the devastating realisation that the whole tragedy could have been avoided if a series of unfortunate events hadn't unfolded, including the fact the United States had not placed Saudi Arabia as a threat to national security.
The report details numerous missed opportunities for the FBI to learn about Khalid al-Mihdhar, Nawaf al-Hazmi and later, Hani Hanjour, who along with two others, hijacked American Airlines Flight 77 and flew it into the Pentagon.
Most shocking of scenarios is the fact al-Mihdhar and al-Hazmi lived with a "reliable" FBI informant while they underwent pilot training and English lessons in San Diego - and yet they continued to slip under the radar.
Even al-Hazmi's name, address and home phone number appeared in the 2000-01 San Diego phone book, despite being on a CIA watch list.
Meanwhile, Hanjour is believed to have had limited contact with the same informant.
A Joint Inquiry into Intelligence Community Activities Before and After the Terrorist Attacks of September 11, 2001 report described the informant as "the Intelligence Community's best chance to unravel the September 11 plot".
Although al-Mihdhar and al-Hazmi were in San Diego for a "significant period of time", the FBI "did not learn of their presence there until after September 11, 2001".
"After September 11, much would be learned about al-Hazmi and al-Mihdhar's time in San Diego and the Intelligence Community's missed opportunities to find and investigate them before the terrorist attacks in which they participated," the reports states.
"The time that al-Hazmi and al-Mihdhar spent in San Diego was an opportunity during which the FBI could have obtained information about them but did not.
"The boarders in the asset's home were in a position to put the asset and the information he supplied to the FBI in jeopardy."
SO WHAT THE HELL HAPPENED?
Though the FBI had established counter-terrorism as its "top priority" at the time, its San Diego office "was continuing to pursue drug trafficking as its top priority in 2001".
"While the FBI made counter-terrorism its top priority on paper, the FBI took few steps to ensure that field offices complied with this directive."
In May 2000, al-Mihdhar and al-Hazmi rented a spare room within the Lemon Grove residence of Abdussattar Shaikh, co-founder of the Islamic Centre of San Diego and a local Muslim leader.
Six years earlier in 1994, Mr Shaikh was recruited by the San Diego branch of the FBI under an agent who the OIG refers to as "Stan", but has since been named as Steven Butler.
"[Mr Shaikh] worked as an informational source, providing to the FBI information acquired in his normal daily routine," the report reveals.
"He normally was questioned about specific individuals who were under investigation by the FBI, although he occasionally volunteered information that he thought might be relevant."
According to FBI interviews with Mr Butler, he said "he thought that the asset [Mr Shaikh] had good judgment about which individuals might pose a threat and that his reporting had been 'consistent' over the years."
Mr Butler would talk to Mr Shaikh at random throughout the years; sometimes several times a day, sometimes it would be months between chats.
When interviewed by the FBI after the attacks, Mr Butler revealed Mr Shaikh had told him that "two Saudi national visitors were residing in a room at his residence".
Mr Shaikh provided "little other identifying information" apart from their first names, Nawaf and Khalid.
"[Mr Butler] contended that he had asked the asset for the boarders' last names but never received them and did not follow up. He said that the asset told him that his boarders were in the US on valid visitors' visas, and they planned to visit and to study while they were in the country.
"In addition, [Mr Butler] said that the asset told him that he believed that the two boarders were good Muslims because of the amount of time that they spent at the mosque."
Mr Shaikh told Mr Butler the men were "quiet tenants who paid their rent" and would often "go outside when using their cellular telephones", which in retrospect "he found this behaviour to be suspicious".
"According to [Mr Butler], the asset did not describe his boarders as suspicious or otherwise worthy of further scrutiny. [Mr Butler] reported that he never obtained al-Hazmi and al-Mihdhar's full identities from the asset and that he did not conduct any investigation of them."
'WE DIDN'T KNOW WE WERE LOOKING FOR THEM'
The problem was, even if Mr Butler had received the men's full names and attempted to investigate the pair, the CIA didn't alert the FBI to their presence in the United States until they were placed on a watchlist a month before the attack in August, 2001, despite having moved to the country in January, 2000.
By this stage, they had already left San Diego.
"How were we supposed to find them when we didn't know we were looking for them?" Bill Gore, who was head of the San Diego FBI office at the time of the attacks said in 2005, in response to a US Justice Department report on the matter.
Mr Butler "also asserted that he was prohibited from further pursuing the information about al-Hazmi and al-Mihdhar, including documenting the information that he had obtained, because of the Attorney General Guidelines" in effect at the time.
The guidelines were designed to ensure that the FBI opened preliminary inquiries and conducted investigations only if the required predicating information was present.
"Most FBI agents in the field did not have direct access to the shared Intelligence Community database that did contain some of the information on al-Hazmi and al-Mihdhar, such as the NSA information," reads the report.
"Field agents could not access, let alone conduct research, on this system. As a result, even if the New York and San Diego agents wanted to search for relevant information about al-Hazmi and al-Mihdhar, any sensitive or highly classified information obtained from the NSA and CIA could not be stored in the one system that they used.
"Because there were no allegations or information provided to [Mr Butler] that al-Hazmi and al-Mihdhar were terrorists or agents of a foreign power, we agree that [Mr Butler] did not have sufficient information to open a preliminary inquiry and actively investigate al-Hazmi and al-Mihdhar."
Though, "if the FBI had documented their presence in San Diego, it would have provided additional investigative leads that could have aided the New York FBI in locating them in August 2001".
'A BOMB HAD FALLEN ON ME'
After the September 11 attacks, Mr Shaikh told the FBI he had "no indicators" al-Hazmi and al-Mihdhar were planning the hijacking. He only realised his former housemates were involved in the attack when he heard their names on the radio three days after the attacks.
"I had to stop my car," Mr Shaikh told the New York Times in 2001.
"My whole body was numb, like my heart was stopping and a bomb had fallen on me. I felt betrayed, very sad and very hurt.'
"I knew these people. Khalid showed me pictures of his baby daughter. Nawaf asked me to help him find a Mexican wife. He liked this country."
But his neighbours spied a conspiracy, telling The San Diego Union-Tribune they "saw several other hijackers visiting Shaikh's home" after identifying their faces from photos in the newspaper.
While Mr Shaikh "consistently maintained" he had no suspicions of the pair's intentions, the results of a polygraph test conducted by the intelligence agency came back "inconclusive".
Yet based on its investigation, "the San Diego FBI concluded that the informational asset had not been complicit in plotting the attacks".
Mr Shaikh declined to be interviewed by both the OIG and Joint Intelligence Committee, asserting his Fifth Amendment privilege.
"The asset indicated through his attorney that if subpoenaed by the committee, he would not testify without a grant of immunity," a footnote in the OIG report read.
Mr Butler declined an interview with the OIG but subsequently testified in a closed session before the Joint Intelligence Committee.
In July 2003, Mr Shaikh was given a $100,000 payment and closed as an asset.