Within a few days of the terror attacks that killed nearly 3,000 people on Sept. 11, 2001, FBI agents identified an important possible suspect: a middle-aged Saudi graduate student who provided crucial help to two of the al-Qaida hijackers as they settled in San Diego early the year before.

The Saudi, Omar al-Bayoumi, had since moved to England, and he was arrested there at the FBI’s request. A police search of his home and office yielded a trove of evidence that quickly deepened investigators’ suspicions about Bayoumi and his dealings with the terrorists. But what happened to much of that evidence after it was delivered to the FBI remains a mystery.

Bayoumi was questioned and released by British police and later returned to Saudi Arabia. For years thereafter, the FBI would insist he wasn’t really a suspect at all — that Bayoumi had met the hijackers by chance, had helped them unwittingly and seemed nothing like a militant Islamist.

As the 25th anniversary of the 9/11 attacks is marked Friday, questions about how the FBI handled Bayoumi’s case have only grown, driven in part by evidence that was seized from him in 2001 but not made public for years or even shared with field agents who sought to investigate his role.

That evidence and other FBI information, now filed in a federal lawsuit against the Saudi government by victims of the attacks, has undercut both Bayoumi’s claims of innocence and the FBI’s profile of him as a genial, pro-American civil servant. While he was living publicly as a student, it shows, Bayoumi worked closely with Saudi religious officials, operated as a Saudi intelligence asset and collaborated with several clerics who would later emerge as militant Islamists tied to al-Qaida.

This week, representatives of the 9/11 families are again demanding an end to what they have called the Justice Department’s silence about what happened to the Bayoumi evidence and why he and other Saudis were not more aggressively pursued. The families have also questioned whether the Trump administration — which has strengthened U.S. diplomatic and business ties to the Saudi royal family — is committed to pursuing the truth about the possible role of Saudi officials.

“We still have not been given the full picture of what happened,” said Kathleen Zapata, whose father, Joseph Coppo, was killed in the World Trade Center. “Why are we having to fight against our own government simply to get answers about the most horrible attack ever on American soil?”

A White House spokesperson said, “President Trump is leading the most transparent administration in history, and his team is working diligently on identifying records that are responsive to these families’ request.” A Justice Department spokesperson declined to comment, noting that many of the officials involved in the early 9/11 investigation no longer work for the government.

While former Presidents Bill Clinton, George W. Bush, Barack Obama and Joe Biden are expected to attend a ceremony at the 9/11 memorial in New York, President Donald Trump has said he will commemorate the anniversary separately at the Pentagon.

The Saudi government has long denied any role in the attacks, often citing FBI and CIA statements that the royal family was an enemy of al-Qaida and its Saudi-born leader, Osama bin Laden. A spokesperson for the Saudi Embassy in Washington did not return messages asking for comment, including a request to speak to Bayoumi.

From San Diego to Dulles International Airport

A ProPublica examination of how the FBI handled the Bayoumi evidence shows that questions surrounding the Saudi connections to the 9/11 plot date to Jan. 15, 2000, when the first two hijackers landed in Los Angeles on a flight from Bangkok.

The CIA had followed the two men, Nawaf al-Hazmi and Khalid al-Mihdhar, as they met with other Qaida operatives in Malaysia days before. But the agency said it lost track of them when they flew to Thailand and then entered the United States using their real names and Saudi passports. (The CIA did not inform the FBI of their presence in the United States until Aug. 24, 2001, according to the report of the 9/11 Commission.)

Unlike several Qaida operatives who led the hijacking teams, Mihdhar and Hazmi spoke no English and knew almost nothing of life in a Western society. The mastermind of the plot, Khalid Sheikh Mohammed, told CIA interrogators he advised them to seek help from Muslim communities in California, according to the 9/11 Commission. But the commission and many U.S.........

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