The documents, kept in a secure room in the basement of the US Capitol, contain information from the joint congressional inquiry into “specific sources of foreign support for some of the September 11 hijackers while they were in the United States”.
Bob Graham, co-chairman of the bipartisan panel, says the documents point suspicion at the Saudis.
US president Barack Obama has hinted the administration might release at least part of the document.
The disclosure would come at a time of strained US relations with Saudi Arabia, a long-time American ally.
“I hope that decision is to honour the American people and make it available,” Mr Graham told NBC’s Meet the Press.
“The most important unanswered question of 9/11 is, ‘did these 19 people conduct this very sophisticated plot alone, or were they supported?’”
Tim Roemer, who was a member of both the joint congressional inquiry and the 9/11 Commission, and has read the secret chapter three times, described the 28-page chapter as a “preliminary police report”.
“There were clues. There were allegations. There were witness reports.
"There was evidence about the hijackers, about people they met with — all kinds of different things that the 9/11 Commission was then tasked with reviewing and investigating,” the former Democratic congressman from Indiana said.
Fifteen of the 19 hijackers were citizens of Saudi Arabia.
The Saudi government says it has been “wrongfully and morbidly accused of complicity” in the attacks, is fighting extremists and working to clamp down on their funding channels.
But the Saudis also say they would welcome declassification of the 28 pages because it would “allow us to respond to any allegations in a clear and credible manner.”
The pages were withheld from the 838-page report on the orders of the then president George W Bush, who said the release could divulge intelligence sources and methods.
However, protecting US-Saudi diplomatic relations also was also believed to have been a factor.
Neither the congressional inquiry nor the subsequent 9/11 Commission found any evidence that the Saudi government or officials knowingly supported those who orchestrated the attacks that killed nearly 3,000 people.
Mr Roemer said many questions remain about the roles of Fahad al Thumairy, an official at the Saudi consulate in Los Angeles who allegedly helped two of the hijackers find housing and transportation after they arrived in Southern California.
He also says he wants to know more about Omar al Bayoumi, who was strongly suspected of being a Saudi spy and was alleged to have been helpful to the hijackers.