Celebrities mark 20 years since 9/11 attacks: 'We must never forget'

Celebrities took to social media Saturday to show that the victims of the 9/11 attacks have not been forgotten 20 years later.

Underwood shared a lengthy post Saturday as she showed the flag flying at half-mast at EH-OK Farm.  "I remember exactly where I was 20 years ago when a friend told me on my way to class what had happened," Underwood wrote in her post. "It was absolutely impossible to comprehend at the time the words that he was saying. To be honest, 20 years later, I still can’t comprehend it."