The US Central Intelligence Agency has taken the first step toward a criminal inquiry into who told the media that it runs secret jails abroad, reports say.
Condoleezza Rice was tight-lipped on the issue of secret jails
The investigation will examine possible leak of classified information, unnamed officials are quoted as saying.
Last week the Washington Post newspaper alleged that the CIA was running detention centres for terror suspects in unnamed Eastern European countries.
The Bush administration has so far refused to comment on the allegations.
On Tuesday Secretary of State Condoleezza Rice averted questions on the issue, saying only that the US was in a "different kind of war" and had an obligation to defend itself.
The BBC's Fergal Parkinson says the repeated refusal by the administration to confirm or deny the reports has fuelled speculation that the secret prisons do exist.
Officials quoted by the Associated Press and Reuters news agencies on condition of anonymity said the CIA had asked the justice department to look into a possible leak.
The department will decide whether to initiate criminal proceedings.
Some governments have already issued denials, including Romania and Poland which were named by New York-based Human Rights Watch as possible hosts for the prisons.
Republican congressional leaders, including Senator Bill Frist and House Speaker Dennis Hastert, had called for a congressional investigation into possible leaks.
Earlier on Tuesday, the US defence department issued a new directive prohibiting physical or mental torture of prisoners.
It says all detainees shall be treated humanely, and specifically bans the use of dogs to intimidate those in custody.
The document sets policy for all US military personnel and contractors, but does not cover other agencies, such as the CIA.