Evan Kohlmann, a New York consultant on terror groups, said the speaker appeared to be Gadahn, who as a teenager converted to Islam in the mid-1990s while in contact with Muslims in Garden Grove, Calif.

Federal officials said two years ago that Gadahn was suspected of going to al-Qaida training camps and serving as a translator for the terror network.

"He's the only known individual of Britain or American background in senior levels of al-Qaida," said Kohlmann, whose globalterroralert.com studies al-Qaida.

The video also contained a long testimonial from one of the London bombers, Tanweer, in which he gave his motives for taking part in the attacks and warned of more to come. Some of it appeared in the edited version broadcast by al-Jazeera.

"For the non-Muslims in Britain, you may wonder what you have done to deserve this," Tanweer said in a thick north English accent.

Britons oppress "our mothers and children, brothers and sisters from the east to the west in Palestine, Afghanistan, Iraq and Chechnya," he said.

"Your government has openly support for the genocide of more than 150,000 innocent Muslims in Fallujah," he added, referring to the west Iraqi town where U.S. troops fought Islamic militants for several weeks.

"You have openly declared war on Islam," he said.

Glancing down at his text, which was off-camera, Tanweer said: "I tell every British citizen to stop your support to your lying British government and to the so-called war on terror. And ask yourselves: Why would thousands of men be ready to give their lives for the cause of Muslims?"

"What you have witnessed now is only the beginning of a series of attacks that will continue and increase in strength until you withdraw your soldiers from Afghanistan and Iraq," he warned.