The location of a number of prisoners being held by the US, including high level al-Qaeda suspects, has become a cause of concern for aid agencies and human rights groups, including the International Committee of the Red Cross (ICRC).
By Jon Manel
BBC Radio 4 Today programme reporter
The arrest last year of Khalid Sheikh Mohammed - the so called al-Qaeda chief of operations - was a major scalp.
His name still appears on the FBI's list of the most wanted terrorists.
But he is the only one of the 22 whose picture is accompanied by the word 'located'.
Khalid Sheikh Mohammed was arrested in March of last year
The question now is, where is he? The Pakistani authorities said that after being interrogated he was handed over to the Americans.
But his whereabouts are a mystery. A US government official told me KSM, as she called him, is at "an undisclosed location".
We also do not know where a number of others, who have been described as high level al-Qaeda suspects, are being held.
The ICRC first publicly expressed concerns about those being held in undisclosed locations four months ago after its president met US Secretary of State Colin Powell and National Security Adviser Condoleezza Rice in Washington.
But Francois Stam, the ICRC's head of operations for North America and much of Europe, has told the BBC that the organisation still has not received any information on these prisoners and is becoming increasingly worried.
"The ICRC has visited Guantanamo and Bagram and has not come across those persons during the course of its visits," he said.
"We simply do not know how many persons we are talking about or how exactly they have been arrested.
"We don't know whether they are being judged, we don't know anything and this uncertainty is a great source of concern."
Trying to find out where they may or may not be is not easy.
There are three locations at which the US has acknowledged holding detainees: Guantanamo Bay, Afghanistan and Iraq, according to Michael Shavers, spokesman for legal issues at the US Department of Defense at the Pentagon.
"There's been speculation of other locations but those are the three which we've acknowledged holding detainees," he said.
He also said it might be worth trying some of the other departments. So I did. I went first to the State Department.
"We're not the ones doing the detaining," I was told.
Then the National Security Council at the White House.
"We do not and would not comment on those matters. Those are intelligence matters. I refer you to the appropriate intelligence agencies," was the reply.
What about the Department of Justice?
And the FBI? After all he's on their website.
"I can't tell you that. It's a security issue."
Up to 600 suspects are being detained at Guantanamo Bay
The CIA wasn't able to help me either. So I turned to Vincent Cannistraro, a former head of counter-terrorism operations at the CIA.
"It's clear that the information about the location where the high value al-Qaeda suspects are being held is very closely guarded," he said.
"We had heard earlier that they were brought to Diego Garcia but that's such a remote place that it probably would have served only for initial debriefing.
"...They may have been taken there because it's an ideal place to isolate someone and cut him off from all sources of information."
This isn't the first time it has been suggested that Diego Garcia may have been used. And it would be significant for the UK as the island in the Indian Ocean is a British overseas territory.
It is used as an American defence facility under an agreement signed in the 60s.
A US Navy public affairs spokesman said no detainees are or have been held at the naval support facility in Diego Garcia but could not talk about other parts of the base there.
A British Foreign Office spokesman said: "The US authorities have repeatedly assured us that reports that any suspected terrorists or Iraqi prisoners are being or have been held or interrogated on the island, or on any vessel anchored there, are unfounded."
Wherever they may be, organisations like Amnesty International are concerned about how they are being treated.
The New York Times has alleged that Khalid Sheikh Mohammed has been subjected to an interrogation technique known as 'waterboarding', in which a prisoner is strapped down, forcibly pushed under water and made to believe he might drown.
The ICRC says its negotiations to see these men are ongoing but it is still waiting for a positive response to its requests.