The document, known as "File 17," offers clues to what might be in the missing pages of the bipartisan report about 9/11.
"Much of the information upon which File 17 was written was based on what's in the 28 pages," said former Democratic Senator Bob Graham of Florida, co-chairman of the congressional inquiry.
He believes the hijackers had an extensive Saudi support system while they were in the United States.
Saudi Arabia's Foreign Minister Adel al-Jubeir denies any allegations of Saudi complicity, telling reporters in Washington earlier this month that there is "no there there."
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Former President George W Bush classified the 28-page chapter to protect intelligence sources and methods, although he also probably did not want to upset US relations with Saudi Arabia, a close US ally.
Two years ago, under pressure from the families of those killed or injured on September 11, and others, President Barack Obama ordered a declassification review of the 28 pages. It's unclear when all or some may be released.
File 17, first disclosed by 28pages.Org, an advocacy website, names people the hijackers were in contact with in the United States before the attacks. Some were Saudi diplomats, raising questions about whether Saudi officials knew about the plot.
The 9/11 Commission's final report stated that it found "no evidence that the Saudi government as an institution or senior Saudi officials individually funded" al-Qaida. "This conclusion does not exclude the likelihood that charities with significant Saudi government sponsorship diverted funds to al-Qaida," the report said.