On Sept. 11, 2001, the United States suffered through one of the worst terrorist attacks in its history. Four passenger planes were hijacked, two of which struck the North and South World Trade Center towers, killing over 3,000 people.

Eighteen years later, people of all walks of life — celebrities, politicians and ordinary citizens — have come together on social media to remember the tragedy and honor its victims with the hashtags #NeverForget and #September11. Among the bigger names to share their thoughts is former President Barack Obama, who mourned the lives lost but also remembered “the spirit of unity and togetherness” that powered the American people through this devastating act.

Bill and Hillary Clinton also paid their tributes, with the former President expressing his sadness for those who “lost their lives 18 years ago in New York, Virginia, and Pennsylvania, their loved ones, and everyone who has borne the sacrifices of that day.” Several other individuals have taken to Twitter too, including television personality Ellen DeGeneres, actresses Tamera Mowry-Housley and Mindy Kaling, singer Mariah Carey and the heavy metal band Megadeth.