Nearly 20 years after the most deadly foreign assault on U.S. soil, the American people still don’t have all the answers about whether the Saudi government assisted the mainly Saudi terrorists who planned and carried out the 9/11 attacks. That’s because the FBI continues to classify key details of the 9/11 investigation, citing national security concerns. Democratic senators from the states most heavily affected by the World Trade Center attacks are demanding declassification of that information, as are family members of the nearly 3,000 victims.

They’re right. There’s no reason to continue protecting the Saudi government from whatever embarrassment it might suffer from any revelations.

Family members of the victims are among 1,600 signatories of a letter telling President Joe Biden not to attend any commemoration ceremonies marking the 20th anniversary unless he first declassifies the remaining material. Biden has committed to a limited review but has stopped short of full declassification.

The phrase “national security” can encompass a wide range of justifications and excuses. If access to cheap and plentiful oil might be cut off by Saudi Arabia in retaliation for the declassification, the resulting harm to the U.S. economy could arguably be deemed a national security concern. The Saudis could halt purchases of U.S. military weaponry or withdraw its multibillion-dollar holdings in U.S. stocks. Any number of scenarios could fall under the national security umbrella.

But the central question in all such retaliatory outcomes remains unanswered: What do the Saudis have to hide?

For many decades, American citizens have been led to believe that the Saudi monarchy is a staunch U.S. ally that opposes all forms of terrorism and will do whatever it takes to ensure the stability of Western economies.

But it became clear after the 9/11 attacks that the kingdom, in fact, harbored and protected some of the most radical clerics preaching the same interpretations of the Quran as the leaders of al-Qaida, the Taliban and the Islamic State. Saudi funding helped expand religious schools around Pakistan where young men were indoctrinated in radical religious thought.

The royal family has zero tolerance for dissent. Women were jailed for daring to challenge oppressive rules that prevented them from driving or traveling. Washington Post opinion writer Jamal Khashoggi, a Saudi citizen, criticized Crown Prince Muhammad bin Salman in print. For that, the prince dispatched a team of agents to corral Khashoggi inside the Saudi consulate in Istanbul, where they killed him and dismembered the body. The State Department’s 2020 human rights report on the kingdom is rife with stories of kidnappings, torture and forced disappearances.

That’s how Saudi leaders handle embarrassment. So imagine the kinds of retaliation they are threatening behind the scenes to ensure ongoing secrecy about the kingdom’s involvement or complicity in the 9/11 attacks. Which again raises the unanswered central question: What, exactly, are the Saudis trying to hide?