Adam Yehiye Gadahn, is a 28-year-old American, who the FBI believes attended al-Qaida training camps in Pakistan and served as an al-Qaida translator.
It was the second time Gadahn appeared in the same video with al-Zawahri. In a July 7 video marking the one-year anniversary of the terror attack on London commuters, Gadahn appeared briefly, saying no Muslim should "shed tears" for Westerners killed by al-Qaida attacks.
Gadahn spoke for nearly the entire video, wearing a white robe and a white turban, sitting in front of a desk with a computer and Islamic religious books in a room with a white wall.
Gadahn delivered a lecture on Islam and the "errors" in Christianity and Judaism. He also the United States is losing the wars in Iraq and Afghanistan and told U.S. soldiers they are fighting President Bush's "crusades."
"Instead of killing yourself for Bush ... why not surrender to the truth (of Islam), escape from the unbelieving army and join the winning side. Time is running out so make the right choice before it's too late," he said.
Al-Zawahri gave only a brief introduction to the video, calling on Americans to convert to Islam. "To the American people and the people of the West in general ... God sent his Prophet Muhammad with guidance and the religion of truth ... and sent him as a herald," he said.
Little is known about Gadahn's role in al-Qaida. A Californian who converted to Islam, he disappeared soon after the Sept. 11 attacks. In 2004, the FBI announced it was seeking Gadahn in connection with possible terrorist threats against the U.S., but adding it did not have information linking him to any specific terror activities.
"You know that if you die as an unbeliever in battle against the Muslims you're going straight to Hell without passing 'Go,'" Gadahn said on the video, addressing American soldiers. "You know you're considered by Bush and his bunch of warmongers as nothing more than expendable cannon fodder ... You know they couldn't care less about your safety and well-being."
"We send a special invitation (to convert to Islam) to all of you fighting Bush's crusader pipedream in Afghanistan, Iraq and wherever else 'W' has sent you to die. You know the war can't be won," he said, using Bush's nickname.
Gadahn also urged other Americans to convert to Islam.
"It is time for the unbelievers to discard these incoherent and illogical beliefs," he said. "Isn't it the time for the Christians, Jews, Buddhists and atheists to cast off the cloak of the spiritual darkness which enshrouds them and emerge into the light of Islam?"
Gadahn and al-Zawahri appeared in separate parts of the video, which was released by al-Qaida's production wing, As-Sahab. Gadahn spoke with his face uncovered, resembling FBI photos, with his name and nom de guerre - "Azzam the American" - written in titles in Arabic and English next to him. Arabic subtitles translated his comments.
Besides the July 7 video, Gadahn is believed to be a masked figure who appeared in two previous videos not officially from al-Qaida, one given to ABC television in 2004 and another a few days before the fourth anniversary of the Sept. 11 attacks.
In the 2005 tape, the speaker - who had black cloth draped over his face, leaving only his eyes visible - threatened new terror attacks in Los Angeles and Melbourne, Australia. The 2004 tape praised the Sept. 11 attacks and said a new wave of attacks could come at any moment.