In Afghanistan, the largest CIA covert prison was code-named the Salt Pit, at center left above. (Space Imaging Middle East)

The CIA has been hiding and interrogating some of its most important al Qaeda captives at a Soviet-era compound in Eastern Europe, according to U.S. and foreign officials familiar with the arrangement.

The secret facility is part of a covert prison system set up by the CIA nearly four years ago that at various times has included sites in eight countries, including Thailand, Afghanistan and several democracies in Eastern Europe, as well as a small center at the Guantanamo Bay prison in Cuba, according to current and former intelligence officials and diplomats from three continents.

The hidden global internment network is a central element in the CIA's unconventional war on terrorism. It depends on the cooperation of foreign intelligence services, and on keeping even basic information about the system secret from the public, foreign officials and nearly all members of Congress charged with overseeing the CIA's covert actions.

The existence and locations of the facilities -- referred to as "black sites" in classified White House, CIA, Justice Department and congressional documents -- are known to only a handful of officials in the United States and, usually, only to the president and a few top intelligence officers in each host country.

The CIA and the White House, citing national security concerns and the value of the program, have dissuaded Congress from demanding that the agency answer questions in open testimony about the conditions under which captives are held. Virtually nothing is known about who is kept in the facilities, what interrogation methods are employed with them, or how decisions are made about whether they should be detained or for how long.

While the Defense Department has produced volumes of public reports and testimony about its detention practices and rules after the abuse scandals at Iraq's Abu Ghraib prison and at Guantanamo Bay, the CIA has not even acknowledged the existence of its black sites. To do so, say officials familiar with the program, could open the U.S. government to legal challenges, particularly in foreign courts, and increase the risk of political condemnation at home and abroad.

But the revelations of widespread prisoner abuse in Afghanistan and Iraq by the U.S. military -- which operates under published rules and transparent oversight of Congress -- have increased concern among lawmakers, foreign governments and human rights groups about the opaque CIA system. Those concerns escalated last month, when Vice President Cheney and CIA Director Porter J. Goss asked Congress to exempt CIA employees from legislation already endorsed by 90 senators that would bar cruel and degrading treatment of any prisoner in U.S. custody.

Although the CIA will not acknowledge details of its system, intelligence officials defend the agency's approach, arguing that the successful defense of the country requires that the agency be empowered to hold and interrogate suspected terrorists for as long as necessary and without restrictions imposed by the U.S. legal system or even by the military tribunals established for prisoners held at Guantanamo Bay.

The Washington Post is not publishing the names of the Eastern European countries involved in the covert program, at the request of senior U.S. officials. They argued that the disclosure might disrupt counterterrorism efforts in those countries and elsewhere and could make them targets of possible terrorist retaliation.

The secret detention system was conceived in the chaotic and anxious first months after the Sept. 11, 2001, attacks, when the working assumption was that a second strike was imminent.

Since then, the arrangement has been increasingly debated within the CIA, where considerable concern lingers about the legality, morality and practicality of holding even unrepentant terrorists in such isolation and secrecy, perhaps for the duration of their lives. Mid-level and senior CIA officers began arguing two years ago that the system was unsustainable and diverted the agency from its unique espionage mission.

"We never sat down, as far as I know, and came up with a grand strategy," said one former senior intelligence officer who is familiar with the program but not the location of the prisons. "Everything was very reactive. That's how you get to a situation where you pick people up, send them into a netherworld and don't say, 'What are we going to do with them afterwards?' "

It is illegal for the government to hold prisoners in such isolation in secret prisons in the United States, which is why the CIA placed them overseas, according to several former and current intelligence officials and other U.S. government officials. Legal experts and intelligence officials said that the CIA's internment practices also would be considered illegal under the laws of several host countries, where detainees have rights to have a lawyer or to mount a defense against allegations of wrongdoing.

Host countries have signed the U.N. Convention Against Torture and Other Cruel, Inhuman or Degrading Treatment or Punishment, as has the United States. Yet CIA interrogators in the overseas sites are permitted to use the CIA's approved "Enhanced Interrogation Techniques," some of which are prohibited by the U.N. convention and by U.S. military law. They include tactics such as "waterboarding," in which a prisoner is made to believe he or she is drowning.

Some detainees apprehended by the CIA and transferred to foreign intelligence agencies have alleged after their release that they were tortured, although it is unclear whether CIA personnel played a role in the alleged abuse. Given the secrecy surrounding CIA detentions, such accusations have heightened concerns among foreign governments and human rights groups about CIA detention and interrogation practices.

The contours of the CIA's detention program have emerged in bits and pieces over the past two years. Parliaments in Canada, Italy, France, Sweden and the Netherlands have opened inquiries into alleged CIA operations that secretly captured their citizens or legal residents and transferred them to the agency's prisons.

More than 100 suspected terrorists have been sent by the CIA into the covert system, according to current and former U.S. intelligence officials and foreign sources. This figure, a rough estimate based on information from sources who said their knowledge of the numbers was incomplete, does not include prisoners picked up in Iraq.

The detainees break down roughly into two classes, the sources said.

About 30 are considered major terrorism suspects and have been held under the highest level of secrecy at black sites financed by the CIA and managed by agency personnel, including those in Eastern Europe and elsewhere, according to current and former intelligence officers and two other U.S. government officials. Two locations in this category -- in Thailand and on the grounds of the military prison at Guantanamo Bay -- were closed in 2003 and 2004, respectively.

A second tier -- which these sources believe includes more than 70 detainees -- is a group considered less important, with less direct involvement in terrorism and having limited intelligence value. These prisoners, some of whom were originally taken to black sites, are delivered to intelligence services in Egypt, Jordan, Morocco, Afghanistan and other countries, a process sometimes known as "rendition." While the first-tier black sites are run by CIA officers, the jails in these countries are operated by the host nations, with CIA financial assistance and, sometimes, direction.

Morocco, Egypt and Jordan have said that they do not torture detainees, although years of State Department human rights reports accuse all three of chronic prisoner abuse.

The top 30 al Qaeda prisoners exist in complete isolation from the outside world. Kept in dark, sometimes underground cells, they have no recognized legal rights, and no one outside the CIA is allowed to talk with or even see them, or to otherwise verify their well-being, said current and former and U.S. and foreign government and intelligence officials.

Most of the facilities were built and are maintained with congressionally appropriated funds, but the White House has refused to allow the CIA to brief anyone except the House and Senate intelligence committees' chairmen and vice chairmen on the program's generalities.

The Eastern European countries that the CIA has persuaded to hide al Qaeda captives are democracies that have embraced the rule of law and individual rights after decades of Soviet domination. Each has been trying to cleanse its intelligence services of operatives who have worked on behalf of others -- mainly Russia and organized crime.

The idea of holding terrorists outside the U.S. legal system was not under consideration before Sept. 11, 2001, not even for Osama bin Laden, according to former government officials. The plan was to bring bin Laden and his top associates into the U.S. justice system for trial or to send them to foreign countries where they would be tried.

"The issue of detaining and interrogating people was never, ever discussed," said a former senior intelligence officer who worked in the CIA's Counterterrorist Center, or CTC, during that period. "It was against the culture and they believed information was best gleaned by other means."

On the day of the attacks, the CIA already had a list of what it called High-Value Targets from the al Qaeda structure, and as the World Trade Center and Pentagon attack plots were unraveled, more names were added to the list. The question of what to do with these people surfaced quickly.

The CTC's chief of operations argued for creating hit teams of case officers and CIA paramilitaries that would covertly infiltrate countries in the Middle East, Africa and even Europe to assassinate people on the list, one by one.

But many CIA officers believed that the al Qaeda leaders would be worth keeping alive to interrogate about their network and other plots. Some officers worried that the CIA would not be very adept at assassination.

"We'd probably shoot ourselves," another former senior CIA official said.

The agency set up prisons under its covert action authority. Under U.S. law, only the president can authorize a covert action, by signing a document called a presidential finding. Findings must not break U.S. law and are reviewed and approved by CIA, Justice Department and White House legal advisers.

Six days after the Sept. 11 attacks, President Bush signed a sweeping finding that gave the CIA broad authorization to disrupt terrorist activity, including permission to kill, capture and detain members of al Qaeda anywhere in the world.

It could not be determined whether Bush approved a separate finding for the black-sites program, but the consensus among current and former intelligence and other government officials interviewed for this article is that he did not have to.

Rather, they believe that the CIA general counsel's office acted within the parameters of the Sept. 17 finding. The black-site program was approved by a small circle of White House and Justice Department lawyers and officials, according to several former and current U.S. government and intelligence officials.

Deals With 2 Countries

Among the first steps was to figure out where the CIA could secretly hold the captives. One early idea was to keep them on ships in international waters, but that was discarded for security and logistics reasons.

CIA officers also searched for a setting like Alcatraz Island. They considered the virtually unvisited islands in Lake Kariba in Zambia, which were edged with craggy cliffs and covered in woods. But poor sanitary conditions could easily lead to fatal diseases, they decided, and besides, they wondered, could the Zambians be trusted with such a secret?

Still without a long-term solution, the CIA began sending suspects it captured in the first month or so after Sept. 11 to its longtime partners, the intelligence services of Egypt and Jordan.

A month later, the CIA found itself with hundreds of prisoners who were captured on battlefields in Afghanistan. A short-term solution was improvised. The agency shoved its highest-value prisoners into metal shipping containers set up on a corner of the Bagram Air Base, which was surrounded with a triple perimeter of concertina-wire fencing. Most prisoners were left in the hands of the Northern Alliance, U.S.-supported opposition forces who were fighting the Taliban.

"I remember asking: What are we going to do with these people?" said a senior CIA officer. "I kept saying, where's the help? We've got to bring in some help. We can't be jailers -- our job is to find Osama."

Then came grisly reports, in the winter of 2001, that prisoners kept by allied Afghan generals in cargo containers had died of asphyxiation. The CIA asked Congress for, and was quickly granted, tens of millions of dollars to establish a larger, long-term system in Afghanistan, parts of which would be used for CIA prisoners.

The largest CIA prison in Afghanistan was code-named the Salt Pit. It was also the CIA's substation and was first housed in an old brick factory outside Kabul. In November 2002, an inexperienced CIA case officer allegedly ordered guards to strip naked an uncooperative young detainee, chain him to the concrete floor and leave him there overnight without blankets. He froze to death, according to four U.S. government officials. The CIA officer has not been charged in the death.

The Salt Pit was protected by surveillance cameras and tough Afghan guards, but the road leading to it was not safe to travel and the jail was eventually moved inside Bagram Air Base. It has since been relocated off the base.

By mid-2002, the CIA had worked out secret black-site deals with two countries, including Thailand and one Eastern European nation, current and former officials said. An estimated $100 million was tucked inside the classified annex of the first supplemental Afghanistan appropriation.

Then the CIA captured its first big detainee, in March 28, 2002. Pakistani forces took Abu Zubaida, al Qaeda's operations chief, into custody and the CIA whisked him to the new black site in Thailand, which included underground interrogation cells, said several former and current intelligence officials. Six months later, Sept. 11 planner Ramzi Binalshibh was also captured in Pakistan and flown to Thailand.

But after published reports revealed the existence of the site in June 2003, Thai officials insisted the CIA shut it down, and the two terrorists were moved elsewhere, according to former government officials involved in the matter. Work between the two countries on counterterrorism has been lukewarm ever since.

In late 2002 or early 2003, the CIA brokered deals with other countries to establish black-site prisons. One of these sites -- which sources said they believed to be the CIA's biggest facility now -- became particularly important when the agency realized it would have a growing number of prisoners and a shrinking number of prisons.

Thailand was closed, and sometime in 2004 the CIA decided it had to give up its small site at Guantanamo Bay. The CIA had planned to convert that into a state-of-the-art facility, operated independently of the military. The CIA pulled out when U.S. courts began to exercise greater control over the military detainees, and agency officials feared judges would soon extend the same type of supervision over their detainees.

In hindsight, say some former and current intelligence officials, the CIA's problems were exacerbated by another decision made within the Counterterrorist Center at Langley.

The CIA program's original scope was to hide and interrogate the two dozen or so al Qaeda leaders believed to be directly responsible for the Sept. 11 attacks, or who posed an imminent threat, or had knowledge of the larger al Qaeda network. But as the volume of leads pouring into the CTC from abroad increased, and the capacity of its paramilitary group to seize suspects grew, the CIA began apprehending more people whose intelligence value and links to terrorism were less certain, according to four current and former officials.

The original standard for consigning suspects to the invisible universe was lowered or ignored, they said. "They've got many, many more who don't reach any threshold," one intelligence official said.

Several former and current intelligence officials, as well as several other U.S. government officials with knowledge of the program, express frustration that the White House and the leaders of the intelligence community have not made it a priority to decide whether the secret internment program should continue in its current form, or be replaced by some other approach.

Meanwhile, the debate over the wisdom of the program continues among CIA officers, some of whom also argue that the secrecy surrounding the program is not sustainable.

"It's just a horrible burden," said the intelligence official.