9/11 mastermind Khalid Sheikh Mohammed SMILES and waves during his first appearance in the Guantanamo Bay court in 500 days

Sept. 11 mastermind Khalid Sheikh Mohammed, known as 'KSM,' appeared in court for the terrorist attack along with four other Guantanamo Bay detainees for the first time in more than a year on Tuesday.  

KSM and his co-defendants were said to appear in good spirits, smiling with Mohammed waving to the gallery.

The defendants are charged with crimes including terrorism, hijacking and 2,976 counts of murder for their alleged roles planning and providing logistical support to the Sept. 11 plot. They could get the death penalty if convicted at the military commission, which combines elements of civilian and military law. 

According to the 9/11 commission report, KSM was the 'principal architect' behind the attacks. He was a close ally of Osama bin Laden. 

Mohammed is being tried alongside his nephew,  Ammar Baluchi, who wired money to the 9/11 hijackers from inside the US, alleged hijacking trainer Walid bin Attash, Ramzi bin Shibh and Mustafa Hawsawi, who allegedly drummed up funds for the attack. 

All five defendants answered 'yes' when asked if they understood their rights in the proceedings, according to the Washington Examiner. 

Air Force Lt. Col. Matthew McCall presided over the case, the sixth judge to do so.

The pretrial hearings were the latest attempt to advance a case tied up in legal proceedings. They will run from Tuesday through Friday, before resuming Monday through September 17.

Much of the rest of this week will involve closed, classified meetings attended only by the judge, defense and prosecution - and not the defendants, with next week expected to bring arguments over substantive evidence. 

All five members of al-Qaeda have been assigned military lawyers and civilian human rights lawyers. Female civilian lawyers appeared in hijab and head coverings, while female military lawyers assigned to the defense counsel did not.  

It is unclear how much of the hearings the public will learn due to concerns around classified information and national security. 

No recordings are permitted and no video or audio will be released to the public, but an unofficial courtroom transcript will be released around one day later and a sketch artist will be present. 

The quintet have been held at Guantanamo Bay since September 2006 after several years in clandestine CIA detention facilities following their capture. They have endured waterboarding, sleep deprivation, anal rehydration and other forms of torture during interrogation sessions with the CIA. 

Such torture has majorly stalled proceedings as evidence obtained through torture is rarely admissible in court. Covid-19 caused another massive delay as the hearing was slated to begin in January. 

There is still no official trial date nearly 20 years after the attacks, as the US spends near $10-million per year housing each prisoner at Gitmo. 

Photo posted on the website www.muslm.net on September 3, 2009 allegedly shows Al-Qaeda's Khalid Sheikh Mohammed, alleged organizer of the September 11, 2001 attacks at the Guantanamo Bay detention camp

Photo posted on the website www.muslm.net on September 3, 2009 allegedly shows Al-Qaeda's Khalid Sheikh Mohammed, alleged organizer of the September 11, 2001 attacks at the Guantanamo Bay detention camp

Walid Muhammad Salih Mubarak Bin 'Attash, born in Yemen, is accused of training two of the hijackers to fightMustafa Ahmed Adam al Hawsawi, now 53, from Saudi Arabia, is accused of giving financial backing to the hijackers

Walid Muhammad Salih Mubarak Bin 'Attash is accused of training the hijackers to fight, while Mustafa Ahmed Adam al Hawsawi, now 53, from Saudi Arabia, is accused of giving financial backing to the group

Walid Muhammad Salih Mubarak Bin 'Attash, born in Yemen, is accused of training two of the hijackers to fightMustafa Ahmed Adam al Hawsawi, now 53, from Saudi Arabia, is accused of giving financial backing to the hijackers

Walid Muhammad Salih Mubarak Bin 'Attash is accused of training the hijackers to fight, while Mustafa Ahmed Adam al Hawsawi, now 53, from Saudi Arabia, is accused of giving financial backing to the group

The defense teams are seeking to throw out confessions made to the FBI at Guantanamo after CIA torture under 'enhanced interrogation techniques,' considered by many to be torture. 

Mohammed confessed to planning the 9/11 attacks in March 2007 in a statement to Combatant Status Review Tribunal. He said: 'I was responsible for the 9/11 operation, from A to Z.'

On Sept. 11, 19 hijackers took over four commercial airliners, crashing two into the World Trade Center in New York City, one into the Pentagon and one into a field in Shanksville, Pa. 

A fiery blasts rocks the south tower of the World Trade Center as the hijacked United Airlines Flight 175 from Boston crashes into the building September 11, 2001 in New York City

A fiery blasts rocks the south tower of the World Trade Center as the hijacked United Airlines Flight 175 from Boston crashes into the building September 11, 2001 in New York City

Nearly two decades after the nation's most horrific terror attack of all time, the 9/11 mastermind Khalid Sheikh Mohammed is still awaiting trial

Nearly two decades after the nation's most horrific terror attack of all time, the 9/11 mastermind Khalid Sheikh Mohammed is still awaiting trial 

The US government has spent an estimated $161.5 million housing 9/11 mastermind KSM alone. 

American taxpayers spend roughly between $9.5 and $13 million per prisoner, per year. The prison currently has 40 inmates. That's compared to $78,000 spent per inmate at a "supermax" prison in Florence, Colo., home to some of the highest-risk prisoners in the U.S.

Guantanamo has reportedly cost U.S. taxpayers over $6 billion since its inception. Included in that figure are charter planes to and from the island with few passengers, hundreds of thousands' worth of government devices that are destroyed each year to spills of classified information, Pentagon-funded defense attorneys priced at half a million dollars per year and total legal costs amounting to $60 million, even though Guantanamo has only ever had one finalized conviction. 

In 2019, a top attorney there filed a whistleblower complaint against the prison alleging 'gross financial waste' and 'gross mismanagement.'

The cost has risen dramatically over the years – a 2013 Defense Department report calculated the per-prisoner detention cost at only $2.7 million. 

President Obama sought to close Gitmo, bought failed after backlash at the prospect of transferring terrorists to prisons on American soil. 

President Biden has reversed a Trump-era executive order aimed at keeping Guantanamo open, and launched a review of the facility with aims of closing it before he leaves office.