Bryant Neal Vinas, a Muslim convert planning to blow up the Long Island Railroad, allegedly attended meetings of a group connected to al-Muhajiroun, an organisation banned in Britain.
The continuing influence of al-Muhajiroun will come as a concern to counter-terrorism officials in Britain.
Several members of the group were involved in the 2004 plot to blow up a shopping centre and nightclub using a homemade fertiliser bomb.
Other members of the group, founded in London by the radical preacher Omar Bakri Mohammed, have been involved in a suicide bombing in Israel in 2003, soliciting murder during protests against Danish cartoons of the Prophet Mohammed in 2006 and a fire bomb attack in North London in 2008.
Former members were involved in protests against returning soldiers in Luton last year and an off-shoot, Islam4UK was banned in January after threatening to march along the servicemen’s funeral procession route in Wootton Bassett.
There are now concerns in the US that America has caught what they characterise as the “British disease” of home-grown radicals who are traveling to Pakistan to meet with al-Qaeda operatives.
Vinas’s alleged connections to al-Muhajiroun are revealed by a new CNN documentary that charts his journey from all-American boy to al-Qaeda terrorist, including his involvement with the US group, the Islamic Thinkers Society
The group uses a name associated with al-Muhajiroun, the Followers of Ahl us-Sunnah wal jama'ah (ASWJ), and describes Omar Bakri Mohammed as “a man of global vision.”
US counterterrorism officials told CNN that Vinas attended several meetings of the Islamic Thinkers Society in New York.
They say that one of his contacts in Pakistan was a follower of the organisation called Ahmer Qayyum, a failed actor who returned to Lahore after studying in Manhattan.
Qayyum has denied involvement in terrorism but admitted that he made plans to fly to Pakistan in September 2008 with Vinas and said the American wanted to study Islam and be treated for back pain.
But he added: "I feel proud of him. I respect him. ... If I ever meet him, I'm going to hug him. I'm going to love him for what he did.
"And if my message can reach him, you know, I mean I just want him to stay strong. I don't think the American evil empire is going to last too long now. Inshallah [god willing]."
The Islamic Thinkers Society denied that Vinas attended its meetings and blamed US foreign policy for radicalisation adding: “The Islamic Thinkers Society remains an intellectual, political, and non-violent organisation calling people to Islam and participating in activities through an intellectual and political discourse.”
Vinas had also attended the al-Falah mosque in Queens, New York, which is run by the Islamic missionary group Tablighi Jamaat, a group that has been linked to a number of British terrorists.
When Vinas left the US he gave his friend, Alex Acevedo a book called "Inside the Jihad" about a French spy who claimed to have infiltrated al Qaeda.
"I asked him, when are you coming back?" Acevedo told CNN but his friend told him: "I'm not coming back. I will call you in your dreams."
In Pakistan, Vinas met an associate of the British al-Qaeda commander Rashid Rauf, according to investigators.
Rauf was killed by a missile from an unmanned US drone shortly after Vinas was arrested in Peshawar.
He has admitted charges of conspiracy to murder and providing material support to al-Qaeda.
US terrorist Bryant Neal Vinas connected to British radicals - Telegraph