Irish American man Ken Corrigan was a volunteer firefighter who rushed to the World Trade Centre on 11 September 2001.

He returned there today to remember the many friends he lost that day.

"On a day like this, it's still a nightmare for me," Ken said.

"I wake up with nightmares. We were here when the towers came down and we didn't know if were alive or dead when that cloud came over us," he said.

"Twenty years later it is still so painful."

It is a pain that Ken shared today with those who lost loved ones in the terror attacks.

At the 9/11 Memorial in New York, family members wept as the names of the victims were read aloud. Moments of silence were observed at the exact times the planes struck and the Twin Towers fell.

Former US Presidents Bill Clinton and Barack Obama were there alongside the current President Joe Biden.

He called for unity, describing it as America's greatest strength.

There were similar calls in Shanksville, Pennsylvania where Flight 93 crashed in a field on September 11th twenty years ago.

The man who was president that day, George W Bush, delivered an address focusing on the divisions that currently exist in the US.

He said that the unity the American people showed in the aftermath of 9/11 seems very distant now.

"Malign forces seem at work in our common life that turns every disagreement into an argument and every argument into a clash of cultures. So much of our politics has become a naked appeal to anger, fear and resentment," he said.

"On America's day of trial and grief I saw millions of people instinctively grab for a neighbour's hand and rally to the cause of one another. That is the America I know," he said.

Another former president, Donald Trump, released a video message in which he paid tribute to the first responders and those who lost their lives.

He also attacked Joe Biden over the US withdrawal from Afghanistan.

"We had $85 billion of the finest and most sophisticated military equipment, taken from us without even a shot being fired. The leader of our country was made to look like a fool," Mr Trump said.

There has been much focus on Afghanistan on this September 11th anniversary. The end of America's longest war marks a major shift in US foreign policy.

As the final troops left, Joe Biden said that rather than the threats of 2001, he needed to deal with the threats of 2021.

There is no shortage of new potential dangers out there, both foreign and domestic.

A lot has changed in America over the last two decades but for those who lost loved ones on this day twenty years ago, the pain and suffering remain.