On September 11th, 2001 at exactly 8:46 AM local time, a plane hit the North Tower of the World Trade Center in Manhattan, New York.
Another plane flew into the South Tower just 17 minutes later.
In less than two hours, both 110-storey towers collapsed, killing more than 2-thousand people.
Another plane crashed into the Pentagon near Washington D.C., while another plummeted into a field in Pennsylvania, not reaching its target after passengers fought back against hijackers.
In total, 2,977 people died during nine-eleven.
Twenty years later, the memories of what happened are still raw.
"I think it happened this morning. It's an everyday thing. There isn't a day that goes by that I don't think of something that day. Whether it's a person, whether it's a noise, whether it's a plane flying low."
"People don't understand it's cumulative grief. It doesn't leave you. There's things that trigger it, but people often say to me, well, it's been 20 years, but it never leaves you, it never leaves you."
1,647 victims have been identified but still more than a thousand remain unknown.
Soon after the attacks, then President George W. Bush declared war against terrorism, targeting al-Qaeda leader Osama bin Laden who was eventually killed in 2011 by a U.S. Navy Seal.
However, five suspects including Khalid Sheikh Mohammed, charged with being the mastermind behind the attacks, are still on trial despite first being arrested in 2003.
There have been a number of issues throughout the trial, including suspicions that those charged have been subjected to torture in Guantanamo detention camp.
There has also been an 18-month delay due to the pandemic.
"One of the most important issues in the case is how the torture of these men is going to ultimately affect the trial."
And all this comes with the backdrop of the U.S. withdrawing its troops from Afghanistan after declaring an end to the two decade-long war and the Taliban regaining control and announcing a new interim government earlier this week bringing new challenges to the region.
Kim Cheong-ah, Arirang News.