Story Highlights• NEW: Gadahn placed on most-wanted list; $1 million reward offered
• Californian first American charged with treason since World War II era
• Gadahn, 28, appealed to Americans in al Qaeda videos
• Nicknamed "Azzam the American," he hailed 9/11 hijackers in video
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WASHINGTON (CNN) -- An American al Qaeda propagandist was indicted Wednesday on treason charges, the first person charged with the offense during the United States' war on terrorism, officials said.
Adam Yahiye Gadahn, who has appeared in five al Qaeda videos, is also charged with offering material support for terrorism, U.S. Deputy Attorney General Paul McNulty said.
He has been put on the FBI's Most Wanted Terrorists list, and the State Department is offering a $1 million reward for information leading to his arrest and conviction.
The 28-year-old California native is the first American to face the charge since the World War II era, McNulty said.
"A charge of treason is exceptionally severe, and it is not one we bring lightly," McNulty said at a news conference in Washington. "But this is the right case for this charge."
If apprehended and convicted, Gadahn could face the death penalty.
Nicknamed "Azzam the American," Gadahn is not in U.S. custody and is believed to be living in Pakistan, McNulty said.
McNulty said he believes that Gadahn has been involved in issuing propaganda but not in carrying out any terrorist attacks. (Watch how Gadahn came to be indicted for al Qaeda ties -- 1:59 )
Gadahn has appeared in several al Qaeda messages speaking English and appealing to Americans.
In his latest video appearance, Gadahn called for the world to convert to Islam and praised the hijackers who carried out the September 11, 2001, attacks as "dedicated, strong-willed, highly motivated individuals with a burning concern for Islam and Muslims."
That video, issued days before the fifth anniversary of 9/11, featured both Gadahn and Osama bin Laden's second-in-command, Ayman al-Zawahiri.
In a September 2005 video, Gadahn referred to the 9/11 attacks as "the blessed raids on New York and Washington." According to the indictment, Gadahn, referring to prior attacks in Europe, said, "Yesterday, London and Madrid. Tomorrow, Los Angeles and Melbourne, Allah willing. And this time, don't count on us demonstrating restraint or compassion."
The FBI first put out an alert on Gadahn in 2004, saying he was "being sought in connection with possible terrorist threats against the United States," although the agency said it had "no information indicating this individual is connected to any specific terrorist activities."
FBI Director Robert Mueller last month said Gadahn was of "significant" importance to al Qaeda as a spokesman.
"People have seen his face. It's going to be very difficult for him to become an operative. But he is a contributor," Mueller said.
The charge of treason is rarely used because, in order to win a conviction, the Constitution requires testimony from "two witnesses to the same overt act," or a confession.
While McNulty would not discuss the evidence against Gadahn in detail, he said prosecutors were "very confident" they could meet the requirement for two witnesses, noting that "a number of individuals" could identify Gadahn as the person speaking in the al Qaeda videos.
Wednesday's indictment was handed down by a federal grand jury in Santa Ana, California. Gadahn's last known address in the United States was in Orange County, California.
Gadahn grew up in Riverside County, California, on a goat farm. His father was a rock guitarist before starting the farm with his wife.
Gadahn left when he was a teenager, according to his father. After a heavy-metal rock phase, he turned to Islam in 1995 and wrote about it on the Internet.
"As I began reading English translations of the Quran, I became more and more convinced of the truth and authenticity of Allah's teachings contained in those 114 chapters."
Gadahn's call on Americans to convert came with an explicit warning of what would happen if his call went unheeded.
"Anyone who pays any attention to the messages of the leaders of the jihad, like Sheik Osama bin Laden and Sheik Ayman al-Zawahiri, may God protect them, will know that they have been consistent in inviting the Americans and other unbelievers to Islam and in pressing upon them that they want the best for them. And making it clear to all that we have no choice but to fight those who fight us," Gadahn said.
Gadahn's family had no comment about the latest tape. In the past, his father has said Gadahn moved to Pakistan in 1998 and that the family lost touch with him in 2002.
CNN's Kelli Arena, Stan Wilson and Henry Schuster contributed to this report.
Adam Gadahn was seen in a September al Qaeda video inviting Americans to join Islam "because this could be your last day."