Usama bin Laden (search) has been publicly silent for the longest period since the attacks of Sept. 11, 2001 (search). The question for U.S. intelligence: What, if anything, does it mean?

The terror leader with the $25 million bounty on his head issued two audio statements in December, the last known public word.

He was last seen in a videotaped message to Americans on Oct. 29, 2004, saying the United States could avoid another Sept. 11 attack if it stopped threatening the security of Muslims.

"Any state that does not mess with our security has naturally guaranteed its own security," bin Laden said in a translation of an address aired on Al-Jazeera discussing the 2004 presidential elections.

Since the 9/11 attacks, the longest bin Laden had gone without issuing a new public statement — written, audio or video — was just over nine months. He's now let 10 months pass, and counting.

Two U.S. counterterrorism officials, speaking on condition of anonymity because of the subject's sensitivity, say there isn't evidence to suggest he's dead. The working assumption is that bin Laden is alive, even if he isn't churning out tapes.

Ben Venzke, chief executive at the IntelCenter, a government contractor that does support work for the intelligence community, said terrorism analysts are paying attention.

"This is the first time things have changed in years. Messages have generally come in a consistent pattern, and now they are not," Venzke said. "It is likely that these changes in messaging by Al Qaeda (search) are the result of planning and a P.R. strategy, as opposed to their computer broke."

He noted it was also the first October since 2002 that bin Laden had not delivered a message addressed specifically to Americans.

The terror leader is believed to be hiding in a rugged area along the Afghan-Pakistani border, where the government in Islamabad has little control and tribal loyalties run deep.

Venzke notes there could be a number of factors contributing to bin Laden's public silence. He may have decided to change the messenger. His deputy, Ayman al-Zawahri (search), has been much more vocal, issuing seven messages this year. In years past, he and bin Laden have delivered roughly the same number of messages.

Or the earthquake in Pakistan could have inhibited bin Laden's ability to transmit messages. Or a tape could have been destroyed in the rubble. Yet al-Zawahri has managed to send out a message since the earthquake, calling on Muslims to provide aid.

Bin Laden also could be plotting an attack on the United States and has made a strategic messaging decision to keep quiet in the lead-up to the attack, Venzke said.

In a recent interview, the head of the National Counterterrorism Center, retired Vice Adm. Scott Redd (search), said bin Laden can't communicate with his followers the way he had in the past.

"The more you communicate, the more you try to directly run an organization, the more vulnerable you are," Redd said. "And he is pretty deep in hiding. We know he is not communicating very much."

President Bush rarely mentions bin Laden, who has eluded U.S. capture despite being the most-sought terrorist in the world. Bush did mention him by name in a series of speeches focused on the war in terror last month.

"Al Qaeda's leader, Usama bin Laden, has called on Muslims to dedicate, their 'resources, sons and money to driving the infidels out of their lands,"' Bush said in Norfolk, Va., on Oct. 28. "The tactics of Al Qaeda and other Islamic extremists have been consistent for a quarter-century: They hit us, and they expect us to run."

Public confidence in Bush's handling of foreign policy and terrorism has steadily dropped since a high point after the Sept. 11 attacks. In the most recent AP-Ipsos polling, 46 percent approved of his handling of those issues.

Half of Americans think it's likely that the United States will capture or kill bin Laden, a number that has moved little over the last three years, according to a CNN-USA Today-Gallup poll.