In pictures: The U.S. commemorates the 25th anniversary of 9/11

Key Takeaways

• The attacks occurred on September 11, 2001
• Hijackers flew four passenger airliners, with two crashing into the World Trade Center
• The attacks caused the collapse of the twin towers and damage to the Pentagon
• The attacks were carried out by members of Al-Qaeda
• Commemorations were held at the National 9/11 Memorial & Museum to mark the 25th anniversary