Twenty-five years after the September 11 terrorist attacks, the nation is preparing to mark a solemn milestone.
According to USA Today, nearly 3,000 people were killed on September 11, 2001, when al-Qaida terrorists hijacked four commercial airliners and crashed them into the World Trade Center, the Pentagon and a field outside Shanksville, Pennsylvania.
For millions of Americans, 9/11 remains a defining national tragedy. But nearly 30% of US residents were born after 2001, meaning many know the attacks only through history lessons, memorials and the stories of survivors.
As the nation commemorates the 25th anniversary with ceremonies and tributes, here's what to know about the attacks, who carried them out and the legacy they left behind.
What happened on 9/11?
What began as an ordinary Tuesday morning became one of the deadliest days in American history.
Timeline of the attack (New York time)
07:59: American Airlines Flight 11 takes off from Boston for Los Angeles
08:14: United Airlines Flight 175 takes off from Boston for Los Angeles
08:20: American Airlines Flight 77 takes off from Dulles International Airport near Washington DC for Los Angeles
08:42: United Airlines Flight 93 takes off from Newark International Airport for San Francisco
08:46: American Airlines Flight 11 crashes into the North Tower of the World Trade Center
09:03: United Airlines Flight 175 crashes into the South Tower of the World Trade Center
09:37: American Airlines Flight 77 crashes into the western side of the Pentagon
09:59: The South Tower of the World Trade Center collapses
10:03: United Airlines Flight 93 crashes into a field near Shanksville, Pennsylvania, after passengers and crew fight back against the hijackers
10:28: The North Tower of the World Trade Center collapses
17:20: World Trade Center Building 7 collapses after sustaining heavy fire and structural damage
Millions watched in horror as the attacks unfolded live on television and both Twin Towers collapsed within hours.
As per the 9/11 Memorial & Museum a total of 2,977 people were killed in 9/11 attacks, not including the 19 hijackers. It remains the deadliest foreign attack ever carried out on US soil.
