US officials believe with a high degree of confidence that a missing Californian man is the self-described American al Qaeda who threatened to kill thousands of his countrymen in a videotape broadcast two weeks ago, ABC reported Tuesday.

Officials concluded that Adam Gadahn was the man on the video after speaking to his family in Southern California, showing the tape to captured al Qaeda operatives, and conducting weeks of technical analysis, the report said.

In the videotape, which was obtained and broadcast by ABC News, the masked man identifying himself only as "Azzam the American" said his fellow countrymen "are guilty."

"After decades of American tyranny and oppression, now it's your turn to die. Allah willing, the streets of America will run red with blood matching drop for drop the blood of America's victims," the man said.

Although all indications are of a positive match, officials cautioned that there is a small margin of error in the analysis and they cannot be absolutely certain of the speaker's identity.

Director of the Federal Bureau of Investigation Robert Mueller said in a May press conference that Gadahn was associated with al Qaeda leader Abu Zubaydah in Pakistan, and that he attended terrorist training camps in Afghanistan.

Gadahn, 26, converted to Islam at the age of 17. He left California six years ago for Karachi, Pakistan, where his parents last heard from him in the months after Sept. 11, 2001.