Video Contains Threat of Major Attack on U.S.

Intelligence officials are unable to authenticate a videotape in which someone who identifies himself as an American threatens an assault on the United States that would "dwarf" the Sept. 11 (search) attacks.

"We cannot verify the authenticity of this tape," one official told FOX News after the preliminary technical analysis presented inconclusive results. One source said the verification process had been complicated because the tape is a copy, not an original.

The tape was handed to the Central Intelligence Agency and National Security Agency by ABC News two days ago, multiple government sources told FOX News. ABC is said to have received the tape last weekend from a terrorist source in Waziristan, a region in Pakistan near the Afghan border where the Taliban and Al Qaeda (search) are believed to be holed up.

ABC said they would not air the tape until there was some evidence it was authentic.

The speaker in the video is disguised but speaks "alarmingly" fluent English with a slight "international" inflection, sources said. The speaker, who is shrouded and wears dark glasses, identifies himself as an American, praises the Sept. 11 attacks and engages in other "classic" Al Qaeda rhetoric, officials said. The man identifies himself as "Assam the American," a law enforcement source said.

Officials said they did not yet know if the tape was actually from Al Qaeda or even if the speaker actually had any ties to terrorism. None of the information in the video is specific enough to establish a time frame for its production, though the speaker references the conflict in Sudan, the Sept. 11 Commission and, quite bizaarely, the debate in Massachusetts over same-sex marriage.

Some U.S. officials speculated that the voice might belong to Adam Yahiye Gadahn (search). Gadahn, who also goes by the names Adam Pearlman and Abu Suhayb Al-Amriki, is a 25-year-old Southern Californian fingered by the FBI for possible ties to Al Qaeda.

In May, the FBI announced that Gadahn had ties with senior Al Qaeda lieutenant Abu Zubaydah (search) in Pakistan, and that he had attended terrorist training camps in Afghanistan and served as a translator for the terror network.

"Adam Yahiye Gadahn is being sought in connection with possible terrorist threats against the United States," the FBI said in an announcement in May. "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. He should be considered armed and dangerous."

"He is known to have performed translations … as part of the services he has provided to Al Qaeda," the FBI said.

Gadahn, whose whereabouts are unknown, is believed to have knowledge of an imminent attack on the United States and is being pursued by federal officials.

But officials said Wednesday that Gadahn is "just one possibility" as the tape's speaker. One factor complicating the identification is that the speaker's face is completely masked.

FOX News' Bret Baier, Ian McCaleb and Anna Persky contributed to this report.