We will never forget the lives lost on September 11th at the World Trade Center, the Pentagon and on United Flight 93 in Shanksville, Pennsylvania. USA TODAY

Though we do not know their full story, we know their sacrifice.

SHANKSVILLE, Pa. — As horrific as the 9/11 attacks were, they would have been even worse if not for a heroic group of everyday airline passengers whose remains rest here amid a common field of wildflowers and hemlock groves.

Eighteen years ago on Wednesday, radical Islamic terrorists, trained in Afghanistan as part of Osama bin Laden’s al-Qaida network, hijacked four commercial jetliners and aimed them at the emblems of America’s financial and military might.

Two destroyed the twin towers of Manhattan’s World Trade Center. The third smashed into the Pentagon. And the fourth — United Airlines Flight 93 — crashed, upside down, at 563 miles per hour outside this small town, just 20 minutes flying time from the nation’s capital.

Lingering questions

What was the ultimate target of Flight 93? That remains one of the enduring mysteries of 9/11.

The authoritative 9/11 Commission report notes that the hijackers had turned the navigation to guide the Boeing 757 toward Washington. But the report is inconclusive about the final destination: The objective of Ziad Jarrah, the pilot among the hijackers, “was to crash his airliner into symbols of the American Republic, the Capitol or the White House.”

Have we learned more since the report was issued?

“There is no new evidence so far as I am aware,” Chris Kojm, who served as the commission’s deputy director, tells me in an email. “Bin Laden wanted the hijackers to hit the White House. (Ringleader) Mohamed Atta was focused on the Capitol because it is an easy target to find. We really don’t know what Ziad Jarrah and Flight 93 planned to hit.”

In either case, the decision of the unarmed Flight 93 passengers to fight back against their hijackers spared many lives and prevented the even greater blow to America’s psyche that smashing a center of U.S. government would have brought.

To anyone who lived through 9/11, the story of Flight 93 is familiar. But a whole new generation has been born since 2001. The Flight 93 National Memorial serves as both a moving history lesson and a reminder of a time when America was more unified.

The memorial that enshrines the sacrifice of 40 people

The first thing you notice about the 2,200-acre site is how quiet it is. Four days before the 18th anniversary, gentle breezes turn the wind turbines in the distance. The mostly clear sky is reminiscent of the deep blue one on 9/11.

“You’re on holy ground. It’s goose bumps every time,” says Maryann Brett of nearby Johnstown, Pennsylvania, as she stands on the observation platform, looking out at the crash site and memorial plaza. “This is peaceful. … You can’t be divided when you are here.”

An exhibit inside the visitors’ center relates the story of Flight 93. It tells how the flight took off 25 minutes late because of air traffic delays in Newark, New Jersey. How the four hijackers, seated in first class, took control at 9:28 a.m. over eastern Ohio. How passengers used Airfones to contact loved ones and learned of the other attacks. How, in one heartbreaking recording, a passenger left the code to her safe on an answering machine. How an Airfone operator heard passenger Todd Beamer, who worked for Oracle, tell his fellow flyers, “Are you ready? OK. Let’s roll.”

And how, after a desperate struggle, at 10:03 a.m. Flight 93 came down, killing all 33 passengers and seven crew members.

“They made the ultimate sacrifice to help us,” says Craig Sutherly, a first-time visitor from Ada, Ohio, as he stands in front on the Wall of Names. Nearby, a 17-ton sandstone boulder marks the site of impact.

Here in the Laurel Highlands, politics and partisan bickering seem far away. You can’t help but wonder what might have been if not for the courageous actions of the Americans aboard Flight 93. You can’t help but yearn for the sense of unity that fleetingly brought the nation together after the 9/11 attacks.

And you can’t help but regret that it takes a devastating tragedy to overcome what separates us.

Bill Sternberg is the editor of the editorial page. Follow him on Twitter: @bsternbe