September 11, 2001 changed the United States--together, 9 and 11 are no longer just random numbers in history. The digits bring a profound impact to the country and to the world at large.

This year, celebrities paid tribut to the thousands of lives lost due to the 9/11 attacks through tweets. Following hash tags such as #NeverForget, the social media has become an instrument to remind people of the loss that came with the catastrophy.

Taylor Swift shared that visiting the memorial this year is something she will never forget. She regarded 9/11 as "a day we will always remember with tears in our eyes."

"I will never forget this day. The world changed forever but the Freedom tower will always remind us how strong we are #nyc #911 #neverforget," Rachel Zoe said.

Love is Lady Gaga's advice, "Today we remember and honor all the lives + families ruined by the tragedy of 9/11. Practice love in their memory today #WeWillNeverForget."

E! Online lists other celebs who paid tribute, including Eva Longoria, Sarah Michelle, Kris Jenner and Chris Brown. Perez Hilton said Lindsay Lohan, Lauren Conrad and Carson Daly also expressed their sentiments.

President Barack Obama himself has also tweeted, "Thirteen years after small and hateful minds conspired to break us, America stands tall."

On this date, Americans and the world commemorate the tragic events and the precious lives of around 3,000 people who perished. Out of nowhere, planes crashed, cities exploded and chaos surrounded.

Scholastic recalls how terrorists attacked the United States, hijacking four airplanes in mid-flight. New York City witnessed ruins as two of the planes hit two structures of the World Trade Center.

A part of Pentagon, USA's military headquarters in Arlington, Virginia, was also attacked. The last plane destroyed Shanksville in Pennsylvania.

19 people built the conspiracy, added Scholastic. They belonged to the Al Qaeda Terrorist Group, spearheaded by Osama Bin Laden. The group is well-known for coordinating and funding terrorist attacks all over the world. Osama was killed on May 1, 2011 by US troops in Pakistan.

But the fight agains terrorism has not yet ended--with the president confirming this himself. 

Obama detailed the country's efforts to defeat new terrorist ISIL in the recent prime-time address on Sept. 10, a day before the 9/11 memorial. The President said that the objective is clear. He is determined to degrade and ultimately destroy ISIL by a "comprehensive and sustained counter-terrorism strategy."

Hear the complete speech in the video below.