The video deals primarily with the London subway bombings of 2005, but includes a five-minute speech from Gadahn, in English, in which the American lashes out at allegations of war crimes recently leveled at U.S. troops in Iraq.
Mentioned specifically are alleged incidents in Mahmoudiya and Haditha. A U.S. Army private faces charges of raping and killing an Iraqi woman in Mahmoudiya, and the U.S. military is investigating allegations that Marines killed 24 civilians in Haditha in November.
In the video, Gadahn says the incidents represent a pattern of U.S. and British brutality toward Muslims, and he says he saw U.S. soldiers kill civilians in Afghanistan.
"They have murdered thousands of Afghan civilians," he says. "I have seen it with my own eyes, carried the victims in my own arms. You name it, they have bombed it."
The video was broadcast Thursday on the Arabic-language television network Al-Jazeera.
Gadahn has appeared in previous al Qaeda videos wearing a mask, but his face is uncovered in the new video.
Gadahn is wanted in connection with possible terrorist threats against the United States, according to the FBI.
"Although the FBI has no information indicating this individual is connected to any specific terrorist activities, the FBI would like to locate and question this person," the agency's Web site states.
He has several aliases, including Abu Suhayb Al-Amriki, Abu Suhayb, Yihya Majadin Adams and Adam Pearlman, the FBI states.
According to a manifesto penned in 1995 and posted on a University of Southern California Web site, Gadahn was reared on a California goat farm and home-schooled by Christian parents.
He found Islam after moving into his grandparents' Santa Ana home, where he began exploring Internet message boards about religion and reading English translations of the Quran.
"You see, I discovered that the beliefs and practices of this religion fit my personal theology and intellect as well as basic human logic," Gadahn wrote in the essay.
The majority of the videotape deals with the London bombings and states that two of the suicide bombers received explosives training at al Qaeda camps.
The narrator of the 30-minute tape says that suicide bomber Shehzad Tanweer "received, along with the martyr (Mohammad) Sidique Khan, focused and practical instruction on the manufacture of explosives and their use at the camps of Qaeda al-Jihad."
The tape includes footage of men mixing explosives and carrying out small explosions, but it's not clear if the men in the video are the London bombers.
Khan and Tanweer traveled to Pakistan in October 2004, returning to Britain in February 2005, five months before they carried out the London bombings, along with Germaine Lindsay and Hasib Hussain.
Al Qaeda's deputy leader, Ayman al-Zawahiri, who last year first praised and then claimed credit for the London bombing on behalf of al Qaeda, said Tanweer was motivated by Britain's role in Iraq, Afghanistan, Gaza and the West Bank.
"That's why Shehzad came to the camps of Qaeda al-Jihad and spent a period of time with Mohammad Sidique," al-Zawahiri said on the tape. "Both of them were seeking martyrdom and wished they could carry out a martyrdom operation. They were both very insistent on that, and if the brothers discussed something else they would pay them no heed."
Tanweer, speaking in English on the video, says non-Muslims in Britain may wonder "what you have done to deserve this."
"You continue to oppress," he says. "Your government has openly supported the genocide of over 150,000 innocent Muslims in Falluja."
The video also contains recent clips from bin Laden and Abu Musab al-Zarqawi, who was killed by U.S. troops in Iraq last month.