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Israel's Shin Bet (search) security agency has held Palestinian prisoners incommunicado for weeks at a time at a secret detention center in violation of international law, The Associated Press has learned.

Prisoners say they are blindfolded and kept in black, windowless cells. When they ask where they are, they are told: "On the moon."

Israel refuses to say where the center is located or who is being kept there, but hints foreigners are among the prisoners.

The state's attorney confirmed the existence of the center, known as facility 1391, in a June 9 response to a Supreme Court petition filed by the HaMoked (search) human rights group over missing detainees Bashar and Muhammed Jodallah. But Israel would only say the center was located on a secret army base, arguing that revealing the location would jeopardize national security.

Shin Bet only used the center for a brief period when Israel's incursion into the West Bank in April last year resulted in hundreds of Palestinian arrests and a "shortage of detention places," the response said.

Since then, all Palestinian prisoners have been moved, and the center "is used, if at all, for special circumstances, for detainees who are not residents of the territories," it said. It was not clear which law enforcement agency continues to use the facility.

Both the army and prime minister's office, which is responsible for Shin Bet, refused to comment on the identities and nationalities of prisoners kept there, or what is meant by "special circumstances."

In most cases, Palestinians captured by the army or security agents are documented by the military, the police or the prisons service -- enabling relatives, lawyers or rights workers to locate them. With the secret center, however, detainees' names are not published on any list; during their incarceration, they effectively disappear.

"A secret detention center would be a violation of both the 4th Geneva Convention (search) and Israeli law," said Yael Stein of the Israeli rights group B'tselem. "If no one knows where detainees are being held, then they can do what they want with them. They can torture and abuse them, or even kill them, and no one would know."

Human rights groups say they know of at least seven Palestinians who were held at the center, but say it is impossible to confirm how many have been through there in all.

Former detainees said they were kept in bewildering isolation during their time at the center.

Bashar Jodallah, 50, a clothes salesman from Nablus, was arrested with his cousin Muhammad in November 2002 while crossing into Israel from Jordan. He was held for three months, of which he said 38 days were spent at the secret facility. Muhammad Jodallah, 23, was convicted of being a member of the violent Islamic group Hamas and remains in an Israeli jail.

The two cousins were blindfolded for the trip to the facility, where they were separated and did not see each other again, Bashar Jodallah told AP.

He said he was placed in a small, solitary cell with a concrete bed. "The walls and the ceilings were painted black, with no windows. Only a very faint light came from above, so I could not tell if it was day or night," he said.

Every time he was moved from his cell, Jodallah said he was handcuffed and blindfolded with a combination of dark glasses and a black cloth.

When soldiers entered his cell, he was again forced to put on a blindfold so he would not see their faces. The only face he was allowed to see was that of the prison doctor.

"I kept asking them where I was," he said. "The soldiers told me that I was on the moon, and that no one knew where I was ... Sometimes they would tell me I was in space."

Prisoners were also forbidden to talk to each other.

"Once I heard two prisoners speaking. The soldiers came like crazy, shouting and abusing them," Jodallah said.

While prisoners said they weren't physically harmed, Jodallah said the psychological pressure was immense.

"Not knowing where I was, the color of the rooms, the isolation, really affected me," he said. "I just wanted to die."

Relatives of Muhammad Jodallah said one of his interrogators told him jailed Palestinian uprising leader Marwan Barghouti had sat in the same chair he had and joked that Barghouti was so short, his legs did not reach the ground.

Barghouti, a leader of Yasser Arafat's Fatah faction, is on trial in Israel for attacks that killed 26 people.

Barghouti's lawyer, Jawad Boulos, said his client had been taken blindfolded to a secret interrogation center in the north for five days. While he could not confirm the location, he said Barghouti's description of the facility matched the one given by Jodallah.

Prisoners also reported hearing that Mustafa Dirani and Sheik Abdul-Karim Obeid, two Lebanese militia leaders kidnapped by Israel, were held at the center.

In a sworn deposition, a copy of which was obtained by AP, Dirani described a facility that also matched Jodallah's description.

Israel has held Obeid and Dirani since 1989 and 1994 respectively, to use as bargaining chips for Israeli fighter pilot Ron Arad, who was shot down over south Lebanon in 1986. Arad's whereabouts are unknown.

The two are held under special security laws and have not been tried.

For years, their lawyer, Zvi Rish, appealed in vain for information on their whereabouts.

"I had no idea where they were being held," he said.