The government also knows more today about the 16 hijackers who lived outside California than when the 28 pages were classified in 2003. Much of that information remains secret but should be made public. For example, the F.B.I. for a time claimed that it had found no ties between three of the hijackers, including their leader, Mohamed Atta, and a prominent Saudi family that lived in Sarasota, Fla., before Sept. 11. The family returned to the kingdom about two weeks before the attack. But in 2013, a Freedom of Information Act lawsuit brought by investigative reporters led to the release of about 30 pages from an F.B.I.-led investigation that included an agent’s report asserting “many connections” between the hijackers and this family. The F.B.I. said the agent’s claim was unfounded, and the family said it had no ties to the hijackers. Still, a federal judge in 2014 ordered the bureau to turn over an additional 80,000 pages from its investigation, and he is reviewing those for possible public release.

There is one more thing our government could do to shed light on the attack. For more than a decade, the families of Sept. 11 victims have been litigating against the kingdom and Saudi interests, asserting that they facilitated the murder of their loved ones. With the support of the Justice Department, the Saudis used a 1976 law providing foreign nations some immunity from American lawsuits to block those efforts to secure justice. Now, both the Senate and House of Representatives have unanimously passed a bill, the Justice Against Sponsors of Terrorism Act, that would allow a thorough judicial examination of the Saudi role.

Some might ask, 15 years later, what difference does all this make?

In fact, a lot. It can mean justice for the families that have suffered so grievously. It can also mean improving our national security, which has been compromised by the extreme form of Islam that has been promoted by Saudi Arabia.

But the most important reason is to avoid the corrosive effect that government secrecy can have on a democracy. The nation that denies its people information about what it is doing in their name is a nation slogging down a dark alley of public suspicion toward decline and mediocrity. As Daniel Patrick Moynihan put it, “Secrecy is for losers.”