Five years on, Guantanamo remains controversial
Khalid Sheikh Mohammed, the alleged mastermind of the 11 September attacks on the US and 13 other terror suspects are due at the hearings in the US camp.
They were transferred to Guantanamo after years in secret CIA jails.
This is the first time they have faced any court. But human rights groups say the hearings are sham tribunals.
The hearings are being held with no defence lawyers present, and human rights groups say the panels of three military officials could consider evidence obtained by force.
The hearings do not rule on guilt or innocence, but are the first step towards charging a detainee with crimes.
Officials declined to say which of the 14 would go first or how many have refused to take part in the proceedings.
"We started our first hearing, and the purpose is to determine whether the detainee fits the criteria for designation as an enemy combatant," said Pentagon spokesman Chito Peppler.
"There are a number who want to be present and there are a number that have said they don't want to be present," said Bryan Whitman, another spokesman.
"It's a healthy mix on both sides."
Khalid Sheikh Mohammed, of Pakistani-Kuwaiti origins, who was captured in Pakistan in March 2003, has been described by US President George W Bush as "the man believed to be the mastermind of the 9/11 attacks" which killed about 3,000 people.
Another key suspect is Ramzi Binalshibh, an alleged senior al-Qaeda figure originally from Yemen who was captured in Pakistan in September 2002.
A third man, Saudi-born Abu Zubaydah, who is believed to have been the chief al-Qaeda recruiter, was also captured in Pakistan in 2002.
The group also includes an Indonesian, Hambali, who is accused of planning the 2002 Bali bombings that killed more than 200 people.
They were moved from CIA detention last September.
In the past, initial hearings have been open to outside observers, but the Pentagon decided that these cases were too sensitive to be reported freely.
The BBC's Justin Webb in Washington says that, in particular, there is concern that the men might reveal information about how they were captured.
Edited transcripts of the proceedings will be released later after any information deemed to be classified is edited out.
Lawyers acting for the detainees have said this decision undermines the credibility of the whole process.
'Legal black hole'
Five years after the first prisoners arrived, Guantanamo Bay is soon to see a new phase with the expected start of military tribunals or commissions in March or April.
The camp currently holds about 385 suspects accused of fighting for al-Qaeda, the Taleban or associated militant groups.
It is seen by the Bush administration as a vital tool in the "war on terror". It enables the US to interrogate suspects who are not US citizens and hold them - indefinitely if necessary - in territory it controls but which is not subject to normal US court rules.Critics say it is a legal black hole in which suspects have been abused and face either military tribunals or open-ended imprisonment.