9/11 23rd anniversary: Key moments from history’s deadliest terror attacks — in photos

Nearly 3,000 people were killed as terrorist group Al-Qaeda’s 19 hijackers seized control of four aircraft, sending two of the planes into New York’s World Trade Center, one into the Pentagon in Arlington, Virginia, and the fourth into a field in western Pennsylvania on September 11, 2001. It remains the deadliest terror attack in history.

The September 11 attacks killed 2,977 people and left thousands of bereaved relatives and scarred survivors.

Every year, on the 9/11 anniversary, victims’ relatives gather at the site of WTC Twin Towers -- 9/11 Memorial in New York -- to remember the loved ones they lost in the terror attacks.

The relatives read out victims' names on every anniversary of the attacks.

Hundreds of people are expected to gather again at the 23rd anniversary this year on Wednesday.

Reciting the names of the dead is a tradition that extends beyond Ground Zero.

On September 11 anniversaries, the Pentagon’s ceremony includes military members or officials reading the names of the 184 people killed there.

The Flight 93 National Memorial has victims’ relatives and friends read the list of the 40 passengers and crew members whose lives ended at the rural site near Shanksville, Pennsylvania.

Effects of the attacks rippled around the world and through generations as the US responded by leading a ‘Global War on Terrorism’, which included invasions of Afghanistan and Iraq.

As the complex legacy of 9/11 continues to evolve, communities around the country have developed remembrance traditions that range from laying wreaths to displaying flags, from marches to police radio messages.

US President Joe Biden and Vice President Kamala Harris on Wednesday will observe the 23rd anniversary of the September 11 attacks on the US with visits to each of the three sites where hijacked planes crashed in 2001.

"We can only imagine the heartbreak and the pain that the 9/11 families and survivors have felt every day for the past 23 years and we will always remember and honour those who were stolen from us way too soon," White House spokesperson Karine Jean-Pierre said aboard Air Force One on Tuesday.