The first US citizen to be charged with treason since World War II is said to be the mastermind behind al-Qaeda's propaganda operations in Britain.
Californian Adam Yahiye Gadahn, 28, who last week was charged by the US with treason and aiding terrorists, has produced and appeared in a series of videos that have been disseminated over the Internet, including films urging Muslims in Britain to commit terrorist atrocities against Western targets.
British intelligence believes Gadahn produced a 31-minute video featuring July 7 bomber, Shehzad Tanweer, extracts of which were released by Arab satellite station al-Jazeera earlier this year on the anniversary of the London bombings.
Gadahn has been credited with helping produce anti-US media on behalf of al-Qaeda, and now his more recent attempts to disseminate the organization's message to British Muslims are troubling terrorism experts here.
Gadahn is believed to have been responsible for cutting the footage of Tanweer to make his "martyr's will," although a study of the full video suggests he struggled with the task.
In clips towards the end of the tape, which have not been broadcast by Western media but have been obtained by reporters, Tan-weer is seen becoming increasingly agitated. Stumbling over a script that appears to have been written for him, he is seen to take deep breaths as he points a finger at the camera.
Tanweer -- thought to have been filmed several months before carrying out his bombing mission -- finds reading passages of his will difficult, tripping over the sentences and repeating words. Experts said the language employed in the footage showed how senior clerics within the senior command of al-Qaeda were influencing young impressionable followers like Tanweer and Gadahn to perpetuate the organization's messages.
"Muslim blood has become cheap," Tanweer declares at one stage, before urging Muslims to ask Allah to protect "Sheikh Osama" and to "lift the oppression of Muslims across the earth."
"The likes of Tanweer may not be the sharpest tacks in the box," said Josh Devon, senior analyst with think-tank Search for International Terrorist Entities (SITE), which monitors Islamic fundamentalist media. "This is a guy reading a script; you don't get to say some of the things Tanweer said without coming into contact with senior Koranic scholars."
The broadcast of a small part of the Tanweer footage by al-Jazeera earlier this year drew widespread revulsion. The video from which it is extracted is equally disturbing.
Gadahn's slick video shows a computerized simulation of the King's Cross tube exploding. In the video Gadahn takes great pains to reach out to a British Muslim audience. He is seen claiming that neither Forest Gate-style raids, Belmarsh, Guantanamo or the Mosques and Imams National Advisory Board, which was set up last year to tackle radical Islamic clerics in Britain, will "prevent Muslims from exacting revenge on behalf of their persecuted brothers and sisters."
Gadahn goes on to suggest Britain taught the US to kill Muslims, drawing on its "colonialist history."
"Where's Britain in all of this? Coaching from the sidelines," he said.