Former British resident Binyam Mohamed, second left, who has been held at Guantanamo Bay, steps from a plane Monday at Northolt military base in west London. Mohamed claims he was brutally tortured at a covert CIA site in Morocco. He has been freed from Guantanamo after nearly seven years.

London ? The first Guantanamo detainee released since President Barack Obama took office returned to Britain on Monday, saying his seven years of captivity and torture at an alleged CIA covert site in Morocco went beyond his “darkest nightmares.”

Binyam Mohamed’s allegations — including repeated beatings and having his genitals sliced by a scalpel — have sparked lawsuits that could ensnare the American and British governments in protracted court battles.

Looking frail from a hunger strike, Mohamed, who once was accused by U.S. authorities of being part of a conspiracy to detonate a bomb on American soil, stepped off a charter plane and was whisked away by police, border control agents and immigration officials.

The 30-year-old Ethiopian refugee, who moved to Britain as a teenager, was freed after four hours of questioning.

Attorney General Eric Holder, who traveled Monday to Guantanamo Bay as the Obama administration weighs what is needed to shut the facility, thanked Britain for its cooperation in the case.

“The friendship and assistance of the international community is vitally important as we work to close Guantanamo, and we greatly appreciate the efforts of the British government to work with us on the transfer of Binyam Mohamed,” he said.

Lawyers for Mohamed are seeking secret U.S. intelligence and legal documents they say will prove the Bush administration sent Mohamed to Morocco, where it knew he would be tortured. They claim the documents also prove Britain was complicit in the abuse.

Unlike in the U.S., Britain’s leaders don’t have a past government to blame — Prime Minister Gordon Brown’s Labour Party has been in power for more than a decade.

But the case is also a test for Obama. While he has promised Guantanamo’s closure and an end to torture, he has not yet publicly explained how his government will change the process of extraordinary renditions, which involve sending terror suspects to foreign countries to be interrogated.

CIA Director Leon Panetta has told Congress renditions could continue, but that prisoners won’t be handed over to countries where they are likely to be tortured — which has always been the stated U.S. policy.

The Bush administration’s extraordinary rendition program was much criticized, in part because some prisoners were handed over to countries with documented histories of human rights abuses. Morocco was one such country, according to an Amnesty International report.

The United States refuses to account for the 18 months Mohamed says he was in Morocco.