Accused of plotting the September 11, 2001 attacks, the men are due in a military courtroom on Wednesday, the first time the secretive tribunal has been in session since Trump sworn in less than a week ago.
Renewed focus is on the military prison and the glacial legal process after Trump famously vowed while campaigning that he would load Guantanamo with "bad dudes," and said it would be "fine" if US terror suspects were sent there for trial.
It's been nine years since the United States first charged the 9/11 Five with plotting the September 11 attacks and killing nearly 3,000 people.
A multitude of procedural and legal problems, exacerbated by the logistical challenge of hosting a court in Guantanamo, have slowed the case to a crawl.
"We are just as determined as ever to try these individuals under the rule of law," lead prosecutor Brigadier General Mark Martins said.
"We will do that -- however long it takes."
The general said the government will be ready to begin jury selection in March 2018, but defence lawyers scoff at the idea, saying 2020 is more realistic.
The defendants are alleged mastermind Khalid Sheikh Mohammed, Walid bin Attash, Ramzi Binalshibh, Ali Abd al-Aziz Ali -- Mohammed's nephew -- and Mustapha al-Hawsawi.
One of Obama's first acts as president was to issue an order to close Guantanamo's jail, but he failed to do so in the face of Republican opposition and the reluctance of US allies to take in the detainees.
The remaining prison population is now 41, down from 242 when he took office.
On Wednesday, military judge Colonel James Pohl will consider whether hearings can even happen during this session's two allotted weeks.
Bin Attash's chief lawyer, Cheryl Bormann, broke her arm over the weekend, which prevented her from flying to the US naval base on the eastern tip of Cuba.
Because it's a death penalty case, each defendant has the right to a "learned counsel," or capital expert, during every step of the process.
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