Bucca, located in an isolated desert north of the border with Kuwait, was a school for scores of Takfiris, or Sunni extremists who usually ended up in Al-Qaeda, said Abu Mohammed, freed in 2008 after 26 months behind its bars.

"The illiterate and straight-forward people were the easiest prey for indoctrination," said the 32-year-old resident of Ramadi, the former insurgency stronghold 100 kilometres west of Baghdad.

Opened after the US-led invasion of Iraq in 2003, Camp Bucca was the biggest detention centre in Iraq housing up to 22,000 prisoners in 2007.

At its closing on September 17 this year, there were only 8,000 inmates who were transferred to Camp Cropper in Baghdad and Camp Taji, north of the capital.

"The two suicide bombers and the majority of suspects detained after the twin bombings of August 19 against the foreign affairs and finance departments, which killed 95, were released shortly before from Camp Bucca," a senior interior ministry official told AFP.

"We reached the same conclusion for the double attack of October 25 which left 153 dead," the official said of the almost simultaneous blasts at the justice and public works ministries, after which 73 people were arrested.

In addition, according to an officer, "the Iraqi police belatedly realised that many terrorists from Al-Qaeda were released because they had been detained in American prisons under false names and were not under our review."

Captain Brad Kimberly, spokesman for the US prisons authority in Iraq, told AFP that "to date, we have not received any evidence suggesting a former detainee may be involved in either attack."

Former inmates beg to differ, however.

"The Takfiris educated some newcomers, and they soon became as radical as their masters and justified the murder of policemen and soldiers, calling them ′apostates′," said Haj Ahmad, a 45-year-old who spent four years in Bucca before being released in late 2008.

"The most surprising thing is that the Americans released them after they were fully impregnated with ideology and had become killers... they joined Al-Qaeda when they left the camp," said Ahmad.

Abu Mohammed, a 35-year-old labourer who was arrested after the bombing of a major Shiite Muslim shrine in Samarra in February 2006, offered a similar insight into Bucca′s extremists.

"The Takfiris focused on those around the age of 24. They welcomed them warmly, questioned them about the motives which had led to them being imprisoned, their relationship with religion," said Mohammed, who spent three years at the camp.

"Then they indoctrinated them by encouraging them to grow a beard, dress like the Salafists, including killing policemen and soldiers," he said referring to a group that seeks a purer form of Islam.

Abu Yasser, a teacher aged 45 who was also arrested in Samarra, said he was convinced the Americans were aware of the brainwashing carried out by members of Al-Qaeda.

"The people of Al-Qaeda used to persuade them that suicide operations against the infidels would lead them to paradise. There were youths from Samarra who were quite moderate but did not to speak to me because they considered me a traitor," he said.

But Captain Kimberly insists the allegations are unfounded.

"Actually, our theatre internment facilities are some of the best schools to give detainees the tools they need to succeed and contribute to Iraqi society," he said.

"Detainees who enter our facilities are kept segregated from the more extremist detainees specifically because we want to ensure the low-threat detainees are not influenced by those who have no issue in perpetrating attacks against the Iraqi people or coalition forces," said the US spokesman.