Carrie Underwood is one of the many Americans who are reflecting on the 20th anniversary of the 9/11 terrorist attacks. The country superstar turned to social media on Saturday (Sept. 11) to share a picture of the American flag flying at half-staff at her farm, looking back on that day and sharing words of compassion for everyone whose lives were most directly impacted.

"Our flag is flying a little lower here today at EH-OK Farm," Underwood begins the post, before recalling that she was in school on the day of the terrorist attacks. The singer shares that she was on her way to class when a friend stopped her and told her what had happened.

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"It was absolutely impossible to comprehend at the time the words that he was saying," Underwood writes. "To be honest, 20 years later, I still can’t comprehend it."

Two hijacked planes struck the Twin Towers of the World Trade Center in New York City, and another struck the Pentagon on Sept. 11, 2001. A fourth plane crashed into a field in Pennsylvania before it could reach its intended target. The series of concerted terrorist attacks killed nearly 3,000 people and changed the trajectory of American history forever.

"We must never forget September 11 and how it changed us as a country," Underwood writes, urging her fellow Americans to remember those who died and "continue to pray" for those who lost loved ones and are still grieving.

"And we continue to be thankful to those who were there to help…and are still here to help," she writes, ending her post with "God bless America. #NeverForget."

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