The FBI investigation stems from the interrogation of Feroz Abbasi, from Croydon, south London, who was captured in Afghanistan while fighting for the Taliban and is being held in Camp X-Ray at Guantanamo Bay, Cuba.
Abbasi, 23, a former computer studies student, went to fight in Afghanistan after worshipping at Finsbury Park mosque, where Hamza delivers his firebrand sermons.
He is understood to have told the FBI that he was escorted to an al-Qa'eda camp in Afghanistan by James Ujaama, a black Muslim activist from Seattle who was living in London and also praying at Finsbury Park.