SOMERSET COUNTY, Pa. (ErieNewsNow) - Today marks 21 years since the terrorist attacks of September 11, 2001, the deadliest attack on U.S. soil. 

The sunny Tuesday morning was like any other ordinary day to those who vividly remember, like Stephen Clark, a 37-year veteran with the National Park Service and Superintendent at the Flight 93 National Memorial. 

"Pretty much on the entire East Coast, not a cloud in the sky, just beautiful blue skies,” said Clark. 

On the beautiful Tuesday morning 21 years ago, four commercial flights took off from three east coast airports, one of them was Flight 93. 

Clark says once the South Tower in New York City was struck by Flight 175, everybody became aware that the regular Tuesday morning was anything but ordinary. 

“We basically said, as a nation, we're under attack in some way, shape or form,” said Clark. 

About 45 minutes after departing Newark, four terrorists hijacked Flight 93 around 9:28 and diverted course toward the nation's capital. After phoning loved ones, those on board knew they had to do something. 

"13 people made 37 phone calls, and that's how they were able to learn in real time what happened in New York and then ultimately what happened at the Pentagon, which really galvanized them to come together and do something,” said Clark. 

In a matter of 20 minutes, those on board took a vote, devised a plan and made an assault up the center aisle. Six minutes later, Flight 93 crashed on the grounds of an abandoned strip mine at 563 miles per hour. 

The once-abandoned mine is now a 2,200-acre memorial operated by the National Park Service. “A common field one day. A field of honor forever” is etched in the glass that overlooks the final resting place of the 40 passengers and crew who prevented further catastrophe. 

“And when you think about what they were faced with, and then ultimately the decisions that they've made, if they elected not to do anything whatsoever, they were only 18 minutes flying time to Washington, DC.,” said Clark. “It's my firm belief that they did save the Capitol dome from destruction, our symbol of democracy and our symbol of freedom,” he added. 

Clark says the decision and actions of the regular passengers and crew onboard, who were just trying to get to their destination, were extraordinary. He says the National Memorial encompasses everything special about Flight 93 and the heroes onboard who potentially saved countless lives. From the symbolic layout of the memorial, to the relationships made with the families of Flight 93, Clark says the sacred ground is a special dedication to a special group of everyday strangers, especially on this date each year. 

“When you think of the story of Flight 93, you think of 40 individuals, two pilots, five flight attendants and 33 passengers. Just regular people, people going to work, people traveling, people vacationing and so forth,” said Clark. “There's also a lot of joy because we talk about who they were when they were here with us. Their hobbies, whether they like baseball or dogs and so forth. The relationship with the families to the memorial, and the families to our rangers and our ambassadors, to me, that's what strikes me as one of the most special parts of my job. Watching that interaction and then seeing the response of visitors responding to those stories is really something to behold,” Clark added.