The site consists of individual windowless concrete cells, each lighted by a single light bulb glowing 24 hours a day, where detainees said that their only contact with another human being was at twice-daily interrogation sessions.

The jail’s operation highlights a tension between President Obama’s goal to improve detention conditions that had drawn condemnation under the Bush administration and his desire to give military commanders leeway to operate. In this case, that means isolating certain prisoners for a period of time so interrogators can extract information or flush out confederates.

While Mr. Obama signed an order to eliminate so-called black sites run by the Central Intelligence Agency in January, that order did not apply to this jail, which is run by military Special Operations forces.

Military officials said as recently as this summer that the secret Afghanistan jail and another like it at the Balad Air Base in Iraq were being used to interrogate high-value detainees. And officials said recently that there were no plans to close the detention centers.

In August, the administration restricted the time that detainees could be held at the secret jails to two weeks, changing previous Pentagon policy.

In the past, the military could obtain extensions. The interviewed detainees had been held longer but before the new policy went into effect.

Detainees call the Afghan site the black jail.

"The black jail was the most dangerous and fearful place," said Hamidullah, a spare-parts dealer in Kandahar who was detained in June and who, like some Afghans, doesn’t use a last name. "They don’t let the I.C.R.C. officials or any other civilians see or communicate with the people they keep there. Because I did not know what time it was, I did not know when to pray."

Mr. Hamidullah was released in October, after five and half months in detention, five to six weeks of it in the black jail, he said.

Although his and other detainees’ accounts could not be independently corroborated, each was interviewed separately and described similar conditions. Their descriptions also matched those obtained by two human rights workers who had interviewed other former detainees at the site.

While two of the detainees were captured before the Obama administration took office, one was captured in June of this year.

All three detainees were later released without charges. None said they had been tortured, though they said they heard sounds of abuse going on and certainly felt humiliated and roughly used. "They beat up other people in the black jail, but not me," Hamidullah said. "But the problem was that they didn’t let me sleep. There was shouting noise so you couldn’t sleep."

Others, however, have given accounts of abuse at the site, including two Afghan teenagers who told The Washington Post that they had been subjected to beatings and humiliation by American guards.

All three former detainees interviewed by The Times complained of being held for months after the intensive interrogations were over without being told why. In addition to Hamidullah’s detention of five and a half months, another detainee said he remained at the Bagram prison complex for two years and four months; the other was held for 10 months total.

Neither Pentagon nor White House officials would comment publicly because the existence of the site is classified.

Human rights officials said the existence of a jail where prisoners were denied contact with the Red Cross or their families contradicted the Obama administration’s drive to improve detention conditions.

"Holding people in what appears to be incommunicado detention runs against the grain of the administration’s commitment to greater transparency, accountability, and respect for the dignity of Afghans," said Jonathan Horowitz, a human rights researcher with the Open Society Institute.

Mr. Horowitz said he understood that "the necessities of war requires the U.S. to detain people, but there are limits to how to detain."

The black jail is separate from the larger Bagram detention center, which now holds about 700 detainees, mostly in cages accommodating about 20 men apiece, and which had become notorious to the Afghan public as a symbol of abuse. That center will be closed by early next year and the internees moved to a new larger detention site as part of the administration’s effort to improve conditions at Bagram.

The former detainees interviewed by The Times said they were held at the secret site for 35 to 40 days. All three were sent there upon their arrival in Bagram and eventually transferred to the larger detention center on the base, which allows access to the Red Cross. The three detainees were hooded and handcuffed when they were taken for questioning so they did not know where they were or anything about other detainees, they said.

Mr. Horowitz said he had heard similar descriptions of the jail from former detainees, as had Sahr MuhammedAlly, a lawyer with Human Rights First, a nonprofit organization that has tracked detention issues in Guantánamo Bay, Cuba, Iraq and Afghanistan.

The International Committee of the Red Cross does not discuss its findings publicly and would not say whether its officials had visited the black jail. But, in early 2008, military officials acknowledged receiving a confidential complaint from the I.C.R.C. that the military was holding some detainees incommunicado.

In August, the military said that it had begun to give the Red Cross the names of everyone detained, including those held in the Special Operations camps, within two weeks of capture. But it still does not allow the organization to have face-to-face access to the detainees.

All three detainees said the hardest part of their detention was that their families did not know whether they were dead or alive.

"For my whole family it was disastrous," said Hayatullah, another detainee, who said he was working in his pharmacy in Kandahar when he was captured. "Because they knew the Americans were sometimes killing people, and they thought they had killed me because for two to three months they didn’t know where I was."

The detainees interviewed by The Times said the military had mistaken them for Taliban fighters.

"They kept saying to me, 'Are you Qari Idris?’ " said Gulham Khan, 25, an impoverished, illiterate sheep trader, who mostly delivers sheep and goats for people who buy the animals in the livestock market in Ghazni, the capital of the province of the same name. He was captured in late October 2008 and released in early September this year, he said.

"I said, 'I’m not Qari Idris.’ But they kept asking me over and over, and I kept saying, 'I’m Gulham. This is my name, that is my father’s name, you can ask the elders.’ "

Ten months after his initial detention, American soldiers went to the group cell where he was then being held and told him he had been mistakenly picked up under the wrong name, he said.

"They said, 'Please accept our apology, and we are sorry that we kept you here for this time.’ And that was it. They kept me for more than 10 months and gave me nothing back."

In their search for him, Mr. Khan’s family members spent more than 120,000 Afghanis, about $2,400, a fortune for a sheep dealer, who often makes just 50 Afghanis a day. Some of the money was spent on bribes to local Afghan soldiers to get information on where he was being held; they said soldiers took the money and never came back with the information.

Mr. Hamidullah was similarly mistaken for a Taliban member, he said.

After he was in the black jail, interrogators insisted that he was a Taliban fighter named Faida Muhammad. "I said, 'That’s not me,’ " he recalled.

"They blamed me and said, 'You are making bombs and are a facilitator of bomb making and helping militants,’ " he said. "I said, 'I have a shop. I sell spare parts for vehicles, for trucks and cars.’ "

Mr. Hamidullah credits his release, after five and a half months, to the efforts of his family, a senior Kandahar government official, and tribal elders.

Human rights researchers worry that the jail remains in the shadows and largely inaccessible both to the Red Cross and the Afghan Independent Human Rights Commission, which has responsibility for ensuring humane treatment of detainees under the Afghan Constitution.

Manfred Nowak, the United Nations’ special rapporteur on torture, said the site fell into a legal limbo and so was not clearly governed by the Geneva Conventions.