America is mourning the 20th anniversary of the terrorist attacks on New York and Washington that killed nearly 3,000 people.

Two decades on, images from 11 September 2001 are vividly imprinted in the memory of some – everyone can remember where they were – but now represent a historical event for a new generation born after the atrocity.

In a crystalline blue sky, al-Qaida terrorists hijacked four commercial airliners and crashed two of them into the World Trade Center, a symbol of America’s financial might in downtown Manhattan.

People from all over the world were killed in the initial explosions, jumping to their deaths or being pulverized by the collapsing twin towers, a spectacle that horrified audiences on live television.

The hijackers rammed another plane into the Pentagon, the headquarters of the US military near Washington, tearing a hole in its side. A fourth plane – possibly heading towards the US Capitol – crashed into a field in Shanksville, Pennsylvania, after its passengers heroically fought back.

A total of 2,977 people were killed – 2,753 of them at what became known as “Ground Zero” in New York – a bigger toll than the “day of infamy” at Pearl Harbor in 1941.

President Joe Biden and the first lady, Jill Biden, will attend solemn ceremonies at all three sites on Saturday, September 11 to “honor and memorialize the lives lost”, the White House said. At the 9/11 memorial in New York, at 8.30am, they will be joined by former president Barack Obama, who oversaw the killing of the al-Qaida leader Osama bin Laden in 2011, The Guardian reports.