One week after the last US troops flew out of Kabul in a final, unceremonious act that ended the Afghanistan war, the men accused of helping the hijackers whose attacks on September 11, 2001, triggered the conflict are inching toward trial.

Known as the “9/11 Five” and facing charges that could ultimately lead to their execution, the defendants include Khalid Sheikh Mohammed, the purported mastermind of the 9/11 attacks against the US.

All have been in US custody since 2002 or 2003 and were transferred to the notorious Guantanamo military detention facility in 2006.

The five men are the subject of the longest prosecution in US history, marred from the start by allegations of CIA torture that defence lawyers say renders much of the evidence inadmissible.

The CIA was granted broad latitude in how it questioned terror suspects after 9/11 and developed a series of rough methods euphemistically called “enhanced interrogation techniques” and decried by critics as torture.

Agents had an array of measures at their disposal, including waterboarding, chronic sleep deprivation and force-feeding — either orally or rectally.

The US government says Mr Mohammed, a Pakistani citizen better known as KSM, confessed to masterminding the 9/11 attacks and other crimes including the murder of journalist Daniel Pearl, but observers have questioned the reliability of any “confessions” given under duress.

Mr Mohammed and the four other accused are due in court on Tuesday for a pretrial hearing that could last two weeks.

The men were supposed to stand trial this year but this seems unlikely given pandemic-related restrictions that have limited lawyers' access to the remote naval base, carved from the south-eastern tip of Cuba.

The sprawling base, which the US has leased from Cuba since 1903 for less than $5,000 a year, covers 116 square kilometres and includes untouched mangrove forests, jagged hills and picturesque views of the Caribbean Sea.

The infrastructure appears worn in places, with cracked asphalt and rusted oil tanks. But the base is not without its creature comforts, there's a McDonald's and a Pizza Hut as well as a hotel.

Guantanamo has long symbolised the excesses of former president George W Bush’s “war on terror” and is remembered by many for caging orange jumpsuit-wearing inmates snatched from Afghanistan.

Since the early days of the Iraq and Afghanistan wars, the site has been built out into a sprawling maximum-security prison. Over the years, it has held about 800 prisoners but now only 39 remain.

In Washington, Democrats last month called on President Joe Biden to shut the prison and either release or place on trial in federal courts the remaining detainees. Former president Barack Obama had vowed to close the jail but was thwarted by Republicans.

The case of the 9/11 five has dragged on for nearly two decades. The five suspects were first arraigned in 2008 and then again in 2012. It's dragged on so long, it has outlasted several Judges, prosecutors and defence lawyers.

A group of 75 Democrats signed a letter saying the prison was in disrepair, costly and a two-decade human rights embarrassment to the US.

“The prison at Guantanamo has held nearly 800 prisoners throughout its history but currently holds only 39 men, many ageing and increasingly infirm,” they wrote.

“According to reports, the prison costs over $500 million per year to operate, at a staggering annual cost of $13m per prisoner,” they said, and added that the continued operation of the prison was “a stain on our international reputation and undermines our ability to advocate for human rights and the rule of law".