SAN JUAN, Puerto Rico — After more than five years, the Pentagon has disclosed why it is holding a Saudi nicknamed “the Professor” at Guantanamo Bay, saying he once lived with a September 11, 2001, conspirator and received a stipend from Osama bin Laden.

Shaker Aamer’s lawyer denies the allegations, made after Prime Minister Brown of Britain last week requested the release of the Saudi — who has been an unofficial leader among the detainees — and four other former residents of Britain.

The Bush administration, which has been urging other nations to accept Guantanamo prisoners amid international pressure to close the military jail, said the five detainees — and particularly Mr.. Aamer — are dangerous men.

A senior American official said Mr. Aamer shared an apartment in London in the late 1990s with Zacarias Moussaoui, a confessed Al Qaeda conspirator and the only person in America charged in the attacks, and met with Richard Reid, imprisoned in America for trying to blow up an American passenger jet with explosives hidden in his shoes.

Mr. Aamer also trained in the use of explosives and surface-toair missiles and lived on stipends in Afghanistan paid by Mr. bin Laden, the official, Deputy Assistant Secretary of Defense for Detainee Affairs Sandra Hodgkinson, told the Associated Press.

“He has been involved in a lot of significant terrorist plots,” Ms. Hodgkinson said Wednesday.

Mr. Aamer was one of several men on a prisoner council formed in the summer of 2005 — and soon disbanded by the military — to address detainee complaints about conditions, according to his attorney, Clive Stafford Smith.

Later, Mr. Aamer helped organize a hunger strike that at one point involved more than 100 detainees, Mr. Stafford Smith said. Mr. Aamer, whose English is flawless, became a “spokesman” for the detainees, earning the enmity of his captors, the lawyer said.

“They don’t like Shaker because they think he has got a lot of influence over the prisoners,” Mr. Stafford Smith said, adding that Mr. Aamer has been on a hunger strike for months and has been kept in isolation for nearly two years.

Mr. Aamer, who denies any connection to terrorism, has not been charged. Ms. Hodgkinson said he is not among the 80 or so detainees at Guantanamo who are expected to be prosecuted for war crimes in military tribunals. His supporters are skeptical about the American allegations. “It’s interesting that after 5 1/2 years those allegations are coming out at this moment,” said Moazamm Begg, a British citizen who was held at Guantanamo for more than two years and knew Mr. Aamer before the two were sent to Guantanamo. The two men were held in different sections of the American military prison in Cuba and never saw each other there.

Mr. Begg denied that Mr. Aamer had lived with Moussaoui in London and said he does not know whether he met Reid, who attempted to light a shoe bomb on a trans-Atlantic flight in December 2001. He laughed at the allegation that his friend received money from Mr. bin Laden in Afghanistan.

“I find that really funny because we used to live together in the same house … I know he didn’t have any stipends from anybody,” Mr. Begg said.

The beefy and bearded Mr. Aamer was popular among the guards, and one of them even carried a greeting between Messrs. Begg and Aamer, Mr. Begg recalled.

“They gave him the nickname ‘the Professor’ … because he just has the gift of the gab,” Mr. Begg said in a telephone interview.

Mr. Aamer, who has four children and is married to the daughter of a prominent retired imam in South London, worked as a translator for a law firm in London between 1994 and 2001, his lawyer said. He lived in America for a year in the early 1990s, working at a hotel.

Mr. Begg said he and Mr. Aamer went to Afghanistan to help run a school for girls and that they fled amid the chaos that erupted after America invaded following the September 11 terror attacks. Mr. Aamer was turned over to the American government by the Northern Alliance for a bounty, he said.

In Guantanamo, Mr. Aamer at one point helped nearly two dozen detainees file habeas corpus petitions seeking their release by designating himself as their legally required “next friend,” who could authorize suits filed on their behalf, according to an affidavit filed in his handwriting in a Washington court.

“I am their close friend as a result of being placed with them in Guantanamo,” wrote Mr. Aamer, who was sent to Guantanamo in February 2002. “And I know they want me to act on their behalf as their next friend.”

In January, the commander of the Guantanamo detention center at the time, Navy Rear Admiral Harry Harris, confirmed that Mr. Aamer was a “leader” of the detainees but declined to discuss him in detail.

A detention center spokesman declined further comment on Friday.

Mr. Stafford Smith said the emerging allegations appear to be an attempt to keep Mr. Aamer in custody.

“In my opinion it’s total nonsense,” he said.

But Ms. Hodgkinson said she was disclosing allegations against Mr. Aamer and the four other British residents — Jordanian Jamil el-Banna, Libyan-born Omar Deghayes, Ethiopian national Binyam Mohamed, and Algerian Abdennour Sameur — to underscore “the risk these individuals pose,” though the American government had no plans to try any of them.

Mr. Deghayes, she said, has been associated with a militant group in Libya and has “direct connections” to Al Qaeda operatives in Europe; Mr. Mohamed trained on use of explosives and proposed to Al Qaeda leaders that uranium from hospitals be used in terror attacks.

Mr. Sameur has attended Al Qaeda training courses, while Mr. Banna is suspected of participating in a terrorist organization in Spain, she said.

Mr. Stafford Smith, who represents all the men, denied the allegations and said he might pursue defamation charges against Mr. Hodgkinson in Britain to “stop this verbal torrent of falsehoods.”