Christians Under Attack

The tape, called "Invitation to Islam," runs 48 minutes, expert Laura Mansfield said. Al-Zawahiri speaks for about 4 minutes on the tape, and the American narrates the rest.

Californian Adam Gadahn, wearing a white robe and turban, introduces the message by calling on Westerners to convert. (Watch Gadahn accuse President Bush of not caring about U.S. troops -- 5:42)

Gadahn says that even Americans working with President Bush and British Prime Minister Tony Blair are invited to embrace Islam, but they should hurry.

"We invite all Americans and believers to Islam, whatever their role and status in Bush and Blair's world order," Gadahn says. "Decide today, because today could be your last day."

Mansfield, who is a writer and corporate adviser on the Middle East, Islam and terrorism, said the time reference could indicate an attack is near. Muslims believe that non-believers should be given a chance to convert before they are attacked, Mansfield said.

"This may well be a warning," she said.

The only indication of when the tape was made is a reference to the recent fighting between Hezbollah and Israel, which ended August 14.

Gadahn, also known as "Azzam the American," previously has been featured in al Qaeda tapes and is listed as armed and dangerous by the FBI on its Web site. (Watch why it 'makes sense' to use Gadahn as a spokesman -- 3:29)

He is wanted by the FBI in connection with possible terrorist threats against the United States.

Gadahn appeared on a tape last year on the fourth anniversary of the September 11, 2001 terrorist attacks. He also was on a tape earlier this year, on the first anniversary of the London subway bombings. (Watch story of Gadahn's journey from Orange County to al Qaeda -- 1:39)

Much of his portion carries Arabic subtitles, while the segments in Arabic carry English subtitles. CNN is analyzing the tape.

CNN's Henry Schuster contributed to this report.