GARDEN GROVE, Calif. -- A few things struck Haitham Bundakji about the brown-eyed teenager who introduced himself as "Yahya" at the Islamic Society of Orange County nine years ago. He was shy, earnest -- and very much alone.

When he took the vows that marked his formal embrace of Islam, Yahya had to enlist Bundakji, then the mosque's chairman, along with the imam and another employee -- men he barely knew -- as his witnesses. Unlike most converts, Yahya had come to Islam through reading and research, with no close friends in his new faith to guide the way.

That quickly changed.

Within a year, Yahya -- born Adam Gadahn to parents of Jewish and Catholic heritage -- had fallen in with a group of young men who prayed regularly at the mosque but also picked ugly political squabbles with the placid, middle-class congregation from the suburbs south of Los Angeles.

Bundakji remembers the men as angry, rigidly pious, and hypercritical of any Muslim who adopted Western clothes or manners. But they were also bright, articulate and well educated. "Very convincing," Bundakji surmised, "to someone like Adam Gadahn."

Now, in hindsight, the mosque leader believes he may have witnessed Gadahn's second conversion -- into a radical Islamist.

This spring, more than five years after the Southern California native told his family he was moving to Pakistan, he resurfaced on an FBI list of seven alleged al-Qaida operatives wanted for possible involvement in plots against the United States. Now some intelligence sources say they believe Gadahn is the masked man seen on a videotape released just before the elections, warning that American "streets will run with blood."

Gadahn's parents, who still live on the goat farm in rural Winchester, Calif., where he grew up, say they cannot tell whether the man on the tape is their son, whom they last heard from a few months after the Sept. 11, 2001, terrorist attacks. But they say the FBI's depiction does not resemble the Adam they knew -- a gentle, conscientious boy who enjoyed rock guitar and classical music, who backpacked the High Sierras and traveled to Sweden with a favorite aunt.

"Adam was a very typical teenager," said his aunt, Nancy Pearlman. "There's no story about his upbringing." Pearlman said the family is declining to comment further for now, noting that FBI officials have not issued any public statement linking Gadahn to the video.

FBI investigators have not reached a firm conclusion about whether the person on the videotape is Gadahn, according to an FBI official who spoke on the condition of anonymity because of the ongoing investigation. But the official said the FBI is deeply concerned about Gadahn's whereabouts and suspects that he may be involved in al-Qaida-related activities.

An intelligence official said last month that "we have some confidence, but not certainty," that the voice on the videotape given to ABC News was Gadahn's.

But Bundakji, who banned Gadahn from the Islamic Society of Orange County after the young man struck him during an outburst, said he is "100 percent" certain that Gadahn, now 25, is the man in the video whose features are hidden by a scarf and dark glasses.

"I have no doubts in my mind," Bundakji said. "It's his voice, his gestures."

Gadahn came to the Islamic Society via a circuitous path of adolescent inquiry, part of a family tradition of personal and spiritual exploration.

His father, Philip, was the son of a prominent surgeon of Jewish ancestry. Raised agnostic, he dabbled in the '60s psychedelic rock scene before embracing Christianity and changing his last name from Pearlman to Gadahn, which is derived from the biblical name Gideon. He and his wife, Jennifer, abandoned city life for a 40-acre ranch in a remote part of Riverside County, where he learned how to slaughter goats according to Muslim strictures so he could sell the meat at an Arabic market.

Adam was the oldest of their four children, all of whom were schooled at home and exposed to his parents' nondenominational Christianity. But in his teens, Adam went through a period of questioning. In a widely circulated essay that he posted on the Internet in the mid-'90s, Gadahn said he was briefly "obsessed" with heavy-metal music and then turned to Christian radio stations looking for answers. But he decided he could not believe in the Christian concept of the Holy Trinity and began exploring Islamic Web sites and discussion groups.

While living with his grandparents in suburban Santa Ana, he made his first trip to the nearby mosque in 1995. He introduced himself as Yahya -- John in Arabic. "He was here every day, all day," Bundakji recalled. "He performed the five daily prayers here."

At the time, though, Gadahn was not the center's most troublesome new member. Bundakji, a gregarious man who emigrated to California from Jordan in the late 1960s to attend business school, had grown concerned about a group of seven or eight men who had begun attending prayer services a few years before Gadahn's arrival.

The men -- all in their twenties and thirties, most from Pakistan -- would spend hours at the mosque, praying in a circle and "supposedly studying Islam together," Bundakji said. They wore turbans, long robes and long beards, and they spent a lot of time criticizing other members of the mosque. Bandakji eventually expelled the men from the mosque.

Years later, after the Sept. 11 attacks, Bundakji says he was visited by FBI agents who showed him photos of potential terrorism suspects. He recognized three of them as the former troublemakers. But he did not see or hear of the young brown-eyed man until this May, when reporters tracked him down to ask about a name he had never heard -- Adam Gadahn. He turned on the television news and recognized the photos as the man he knew as Yahya.

"He can be manipulated very easily to feel good about himself," Bundakji said. "He was pushed to do what you saw on tape."