"He has made threats before, but there hasn't been a public utterance for a long time, and for that reason no one is being dismissive of it," said one counterterror official, speaking on condition of anonymity while the tape was still being analyzed.
FBI assistant director John Miller, who as a television news correspondent interviewed bin Laden in the 1990s, said bin Laden appeared to be trying to show he still controls his terrorist network, but that the tape should not alarm Americans.
"We've seen this message before, the demands and threats. In rare instances, tapes have been followed by attacks. But in many more, they haven't been," Miller said.
President Bush was told about the audiotape Thursday, White House spokesman Scott McClellan said.
Associated Press Writer Mark Sherman contributed to this story.