The man at the heart of a $1 trillion (£733bn) lawsuit brought by 9/11 families had extensive links to Saudi government officials and went paintballing with one of al-Qaeda's future leaders, video obtained by the BBC reveals.
The footage, which the FBI had within days of the 2001 attacks, stands in stark contrast to Omar al-Bayoumi's 25 years of denials that he was an extremist and a Saudi spy.
Bayoumi, who claimed to be a student when he was in the United States, is seen in one clip embracing Anwar al-Awlaki, who would go on to lead al-Qaeda in the Arabian Peninsula. In another, Bayoumi and Awlaki are paintballing near San Diego with an official from Saudi Arabia's Ministry of Islamic Affairs.
The videos form part of the 9/11 families' lawsuit against Saudi Arabia and have never been shown publicly before.
The lawsuit argues Saudi Arabia was complicit in the attacks and that Bayoumi aided two of the 19 hijackers who flew planes into the World Trade Center and the Pentagon - with another crashing in Pennsylvania.
Bayoumi was arrested in the UK by the Metropolitan Police days after the attacks and the videos were found among his possessions, along with other crucial evidence.
This included a diagram of a plane with a flight descent equation written alongside.
Multiple FBI agents received copies of the diagram, but it was never shared with the British interrogators, even though it had been couriered to the police station where Bayoumi was being held in custody.
The diagram was also kept from some FBI agents investigating Bayoumi, as well as from the 9/11 Commission - the independent inquiry into the largest mass murder in US history, in which nearly 3,000 people were killed. Bayoumi was never prosecuted.
He and the Saudi government did not respond to the BBC's requests for comment, but both have always denied any involvement in, or prior knowledge of, the attacks. The FBI declined to comment.
After a judge ruled the lawsuit could go ahead in 2018, thousands of documents, as well as videos and photographs, have been released to the court and also declassified through an executive order made by former President Joe Biden.
BBC Panorama and Radio 4 have examined these documents, and spoken to dozens of special agents and investigators, to trace the evidence the FBI gathered about Bayoumi and Saudi Arabia's alleged involvement in 9/11 - and to understand why it was never fully pursued over 25 years.
Our investigation also found:
The plane diagram was sent to the FBI's New York office and was analysed, according to an FBI source. But a US Department of Justice (DOJ) official told the BBC he was never shown a proposal to prosecute Bayoumi based on the evidence, even though "we've convicted people for less"
FBI agents in San Diego, where Bayoumi had lived in the US, repeatedly requested copies of evidence but never received most of it
The San Diego and Los Angeles FBI offices were both blocked from interviewing Bayoumi's Saudi government contacts
Staff on the 9/11 Commission objected to the conclusion that Bayoumi was "unlikely" to have been involved with extremists, prompting the report to attribute this to "our investigators who have dealt directly with him"
Eighteen years later, the FBI would hand the notebook page to the court in the 9/11 families' lawsuit, following a subpoena. The page was made public in 2022.
"It's the mathematical calculation of the crash," Jodi Westbrook Flowers, a member of the 9/11 families' legal team, told the BBC. "When we figured out what it was, we knew that it was the smoking gun that we'd been looking for."
The equation, written in Omar al-Bayoumi's handwriting, indicates the height and distance required for a pilot to guide an aeroplane to a given object on the ground. Underneath it is a small drawing of a plane and a series of numbers.
Last year, the federal judge in the families' lawsuit found the notebook page, on the face of it, "connects Bayoumi with knowledge of the 9/11 Attacks".
In 2021, when questioned about it in court, Bayoumi said he had the diagram because he liked finding equations and could not remember when he had written it.
By the evening of 22 September 2001, copies of the plane diagram were in London in the hands of both the FBI and UK counter-terror police - and just metres from the custody suite where Omar al-Bayoumi was being questioned. Records show they passed through at least three further FBI agents in the following two days.
But here, the mystery begins. The FBI briefed the Met officers to question Bayoumi about the other high-priority exhibits, but the detectives in the custody suite were never given, or briefed about, the plane diagram.
Many FBI agents have told us the immediate aftermath of 9/11 was a round-the-clock frenzy. But it is hard to understand how a piece of evidence of this importance, in a case being handled with considerable urgency, was not put into the hands of the interviewing detectives.
The FBI declined to respond to our questions. The Metropolitan Police told us it could not comment because of the ongoing legal case.
After Omar al-Bayoumi's release, agents continued analysing the evidence against him.
The BBC understands the plane diagram was couriered from London for analysis at the New York Field Office, which was in overall charge of the 9/11 investigation.
But we have not been able to trace any records of this analysis or its conclusions. A 2021 Presidential order required the FBI to declassify files relating to its 9/11 investigations, including into Bayoumi.
However, of the files released, the FBI has only produced one document referring to the diagram, despite its evidential significance, and this was only created 11 years after the diagram's initial discovery in 2001.
The diagram seems to have fallen into a black hole.
It was not passed on to San Diego, where Bayoumi had lived in the USA and where the FBI Field Office was also investigating him.
In fact, San Diego - where agents were interviewing his associates, searching his mosque, analysing his telephone records and bank statements - received almost none of the large volume of evidence seized by the Met Police in Birmingham.
Rick Lambert, who ran the FBI's investigation there, told the BBC his office made repeated requests to New York to send copies of all the evidence. "Their position was, we're in charge of the case; we'll send it to you when we're finished analysing."
Lambert said New York did not comply his office's requests. "I believe there is the potential that had we received that evidence and analysed it, that we would have been able to put the pieces of the puzzle together much more quickly."
Documents show that, in spring 2002, the San Diego office was ultimately able to get copies of a handful of videos by obtaining them not from New York, but from the FBI liaison in London.
Agents who tried to investigate Omar al-Bayoumi encountered other roadblocks.
In October 2001 in San Diego, Lambert and colleagues wanted to interview a number of Saudi officials in the US - many connected to the Ministry of Islamic Affairs - who they suspected had co-ordinated the help Bayoumi gave to two of the hijackers, Nawaf al-Hazmi and Khalid al-Mihdhar.
Because diplomats have a special protected status, the agents needed permission from FBI headquarters and the State Department.
"The answer came back, 'no, you're not going to do that'," Lambert, who is now a consultant for the 9/11 families in their lawsuit, told us.
Agents in Los Angeles experienced a similar outcome the following year, when they applied to interview a Saudi religious diplomat Bayoumi had met immediately after his first encounter with Hazmi and Mihdhar in the city.
"We hit a roadblock so hard, I think we all had airbag burns," said Steve Moore, who oversaw the FBI's 9/11 investigation in LA. Moore says his team made repeated requests to interview the official, but permission never came.
Like the FBI, the State Department declined to comment.
In October 2003, the 9/11 Commission - set up by Congress to provide a "full and complete" account of the attacks - interviewed Omar al-Bayoumi in Riyadh. He had returned to Saudi Arabia in the months after his release by British police.
The interview was conducted, in the presence of Saudi secret police, by the Commission's executive director Philip Zelikow, a former advisor to President George W Bush, along with a senior staffer and an agent from FBI headquarters.
The Commission's notes from the time record that Bayoumi was given a remarkable degree of reassurance during the interview. He "was told that no-one believes he knowingly helped the hijackers carry out their terrorist mission".
Bayoumi indignantly said he was a peace-lover who had become "a victim of the victims". During the interview, the delegation broke away for a multi-course dinner hosted by the Mabahith, the secret police, without Bayoumi.
After the interview concluded, the Mabahith told the American delegation they "would like to see him cleared". The Americans responded that no exoneration would be possible before their report was published.
When the 9/11 Commission Report was published in July 2004, Bayoumi was, in effect, cleared. It described him as "obliging and gregarious" and stated there was no evidence he had extremist views.
But some staff on the Commission were not convinced of Bayoumi's innocence, and thought it likely he knew Hazmi and Mihdhar were al Qaeda operatives.
The BBC can reveal that those staff objected to a line in the report concluding that he was "an unlikely candidate for clandestine involvement with Islamist extremists".
In the end, a compromise was agreed, and this conclusion was attributed to "our investigators who have dealt directly with him and studied his background".
The line was written based on the four and a half hours the Commission delegation had spent with Bayoumi, which included the interruption for dinner.
The Commission never saw the plane diagram, the paintballing video, written evidence of Bayoumi's alleged extremism, or much of the rest of the evidence the Met Police had seized.
Philip Zelikow did not answer the specific questions we asked him. He told the BBC the 9/11 Commission's findings had been subject to two government reviews, in 2015 and 2021, and the Commission had been "open about our suspicions and uncertainties. We were careful about whom to accuse of complicity in mass murder."
The Department of Justice told us it was unable to comment on matters and decisions from 25 years ago, involving staff who no longer worked there.
The head of the FBI's 9/11 investigation, Mary Galligan, has previously stated the Commission had "access to everything".
We tried to contact Omar al-Bayoumi through the Saudi government, but he did not respond. He has previously denied knowing Hazmi and Mihdhar were hijackers, that he was a Saudi spy, and that he had any involvement in 9/11.
The Kingdom of Saudi Arabia did not respond to our request for comment. It has previously said any allegation that Saudi Arabia or its officials are complicit in 9/11 is categorically false.
