Abu Hamza helped pay for former student Feroz Abbasi to attend an al-Qa'eda training camp a court has heard.
An extradition hearing against Hamza at Woolwich Crown Court heard how the 48-year-old preacher financed the 24-year-old's trip to Afghanistan.
Mr Abbasi, from Croydon, spent four years in Guantanamo Bay after being captured allegedly fighting with the Taliban.
He was released from the camp, along with three other Britons, in 2005.
Intelligence services believe Hamza is responsible for radicalising hundreds of young British Muslims and helping them attend terrorist training camps in Afghanistan and Pakistan.
Hamza, who was jailed for seven years last February for soliciting murder and inciding racial hatred during sermons at Finsbury Park mosque, is fighting extradition to the US.
The court heard Hugh Keith, representing American authories, explain how Hamza was the head of the radical Islamist group the Supports of Sharia.
But Alun Jones QC, representing Hamza, said the evidence was unreliable because it may have been obtained under torture.
The hearing continues.
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