A dozen years on, it remains the biggest unsolved mystery connecting the congressional community to the defining moment in 21st-century American history: Were the United 93 hijackers aiming for the Dome when the passengers revolted and forced the plane to crash into a bucolic southwestern Pennsylvania field?

Most people who will go to work on Capitol Hill this Sept. 11 presume there’s no dispute, that the answer is an unequivocal “yes.” I confess I can’t share that certainty and have always wondered why it took such hold.

There’s never been anything close to definitive proof. In fact, one of the few al-Qaida plotters who’s revealed information on the topic suggested all those beliefs could well be wrong.

The truth may never be known for sure, and in many respects it is beside the point. The assumption that the “fourth plane” was aiming for Congress became a truism almost immediately. And the realization that the Hill was entirely unprepared for such an attack — whether it was imminent or not — propelled the wholesale intensification of security that is now an accepted staple of congressional life. It’s also a mixed-blessing shell that insulates the legislative branch from the public it represents.

Still, it remains strangely important for the collective congressional psyche to believe the terrorists had decided the Capitol was a symbol of American democracy and capitalism just as important as the Pentagon and the World Trade Center — an icon of evil meriting even quicker destruction than the White House.

The limited available evidence suggests the opposite.

One of the most important plotters interrogated during the investigation of the Sept. 11 commission, Ramzi bin al-Shibh, said Osama bin Laden made it clear to him and other subordinates that he viewed the president’s house as a more symbolically important target than the Capitol.

Al-Shibh said he conveyed bin Laden’s views repeatedly to Mohamed Atta, the ringleader of the hijacker team. But in several of their conversations, Atta remained resistant to making the White House (“politics,” in their agreed-upon code) the final target — not only because it was relatively small and obscured by foliage but because the Capitol (code-named “law”) protruded above the skyline and would be teeming with potential lawmaker victims one week returned from their August recess.

By two days before the attacks, wrote the commission in a little-noticed footnote to its definitive 2004 report, a classic split-the-difference Washington compromise appears to have been struck between the two al-Qaida operatives: “The White House would be the primary target for the fourth plane and the U.S. Capitol the alternate.”

Bin Laden, of course, is not around to verify that he wanted to physically decapitate the executive branch more than he wanted to cripple the legislative branch.

Whether Atta actually gave his United Flight 93 henchmen permission to make a gametime decision, or instructed them with his preference to go for the easier target, can never be known. He died piloting American Flight 11 into the North Tower in New York. Four other hijackers all perished flying 580 miles an hour into the Shanksville, Pa., dirt at 10:03 a.m. — less than 20 minutes before pilot Ziad Jarrah would have had to decide whether to dive left toward the South Lawn or fly straight toward the West Front.

By that time, on that phenomenally beautiful morning, the evacuation of the Capitol complex was just getting started — too late to prevent mass casualties had the Dome’s interior marble been shattered and its cast iron exterior started to melt.

But the myth that the Hill was targeted for attack was building by the minute. CNN briefly reported a large fire near the Capitol. The rumbling of close-by aircraft engines — belonging, as it turned out, to fighter planes scrambling way too late to do any good — caused panicked screams. So did a large jet flying through the middle of the restricted airspace above the Hill — another piece, it came out later, of the Air Force effort to figure out what was going on.

The myth has lived on ever since. Vice President Dick Cheney gave it voice a few days later, telling a national TV audience that United 93’s “target probably would have been the Capitol” because “it’s big; it’s easy to hit.” The Senate historian’s website says plainly that “the Capitol once again became the target of foreign enemies” on Sept. 11, 2001, part of a string of attacks dating back to the burning of the building by the British in 1814.

And Senate Majority Leader Harry Reid declared flatly, during 10th anniversary ceremonies two years ago commemorating the unusually unified congressional reaction to the attacks, that “we now know” the hijackers were aiming for his workplace rather than the president’s. It’s a claim the Nevada Democrat’s spokesman declined to support at the time, suggesting it might have been a slip into rhetorical excess.

Al-Shibh apparently remains at the Guantánamo Bay prison in Cuba and might have provided more information in recent years. The same can be said of Sept. 11 mastermind Khalid Sheikh Mohammed.

Even if those terrorists have said all they’re going to regarding that score, they might offer this warning to those Hill dwellers who find ironic pride in believing they had a near brush with death 12 years ago: One al-Qaida hallmark is returning to a chosen target as often as it takes until its destructive job is done.