Each event reshaped borders, policies and perceptions, forcing the world to confront the growing reach of extremism in the 21st century. 

From 9/11 to 26/11: 7 deadliest terror attacks that shook the world

'Third tower also collapsed on 9/11': The conspiracy theories and importance of Tower Seven

9/11 attacks and the money trail: How terrorists got funds for the devastating strike

In Pics | Over 1,200 evacuated as fire takes over World Trade Centre in Hong Kong

Some Sept 11 trial secrets may not be secrets anymore

Watch | In a first, New York's World Trade Center lit up for Diwali

New York city's iconic World Trade Center has been lit up to showcase a pretty spectacular digital mural to celebrate the festival of lights, Deepavali.

New York: Spectacular digital mural on World Trade center podium for Diwali

9/11 attacks: 17 new documentaries to watch to commemorate the 20th anniversary

On September 11, 2001, two planes were flown into the famous twin towers of the World Trade Center in New York City. 19 terrorists associated with the Islamic extremist group al-Qaeda hijacked four airplanes and carried out suicide attacks against targets in the United States. The third plane hit the Pentagon just outside Washington, DC, and the fourth plane crashed in a field in Pennsylvania. Over 3,000 people were killed during the 9/11 terrorist attacks. The 9/11 attack triggered major US initiatives to combat terrorism.

What happened on September 11, 2001? | 9/11 attacks

US to mark 18th anniversary of the September 11 attacks

US Senate passes 9/11 victim fund, sends it to President Donald Trump

Al-Qaeda and Islamic State 'know that they will never be able to defeat a nation as great and as strong as America,' he said

Obama on 9/11: Diversity one of America's 'greatest strengths'