WASHINGTON (Reuters) - Nearly one year after President Bush declared Osama bin Laden was wanted "dead or alive" the trail has gone cold and his al Qaeda network, blamed for the Sept. 11 attacks, is almost certainly preparing to strike again, U.S. intelligence experts say. Despite intense efforts to track him and with a $25 million bounty on his head, bin Laden appears to have vanished. Popular theories are that he died in a cave during U.S. bombing of Afghanistan; he disappeared into the lawless tribal region along the border of Pakistan and Afghanistan where supporters are hiding him, or he died from illness or wounds and the few who know are not talking. Bin Laden might also have taken refuge in another country such as Iran, a neighbor of Afghanistan, experts say. U.S. intelligence agencies and the military assume bin Laden and top aide Ayman al-Zawahri are together, alive and in hiding. Intelligence agencies have not picked up communication by followers lamenting bin Laden's death, and none of the al Qaeda members in U.S. custody have said he is dead. "He's such an icon, and if he died it would have filtered out because there would be a great feeling of doom and gloom," said Stanley Bedlington, former CIA counterterrorism analyst. Despite his distinctive height, about 6-feet-5-inches, and an image broadcast around the world, the last time U.S. spy agencies had evidence he was alive was early December. "He's long overdue to make a credible, indisputable appearance. You have a mass of people out there that need to know he's alive," a former intelligence official said. OSAMA SIGHTINGS Claims of bin Laden sightings have not been confirmed. "It's sort of like Elvis sightings -- plenty of them but most of them are wrong," a U.S. intelligence official said. But there are occasional attempts to collect the reward. One Afghan offered a head as proof of bin Laden's death. It was quietly shipped to the United States for testing, but turned out to be of female origin. FBI and military labs have tested a variety of tissue samples from Afghanistan against DNA collected from bin Laden's family, but have found no match, officials say. The United States also has DNA for a few other senior al Qaeda members. The United States was unlikely to declare bin Laden dead without proof, but it was important to establish his fate to prevent his myth from growing in the Muslim world as a hero who fought the West and won, intelligence experts said. Those who know bin Laden's location were either killed with him or are hiding with him, and being "true believers" are unlikely to give him up for money, experts say. "The people who have an inkling may be part of such an ideological and anti-outsider group ... that you could offer them a billion dollars and it wouldn't matter," former CIA Director James Woolsey said. The war on terrorism has destroyed al Qaeda's haven in Afghanistan, disrupted the network with hundreds of arrests around the world and blocked funding. But autonomous small cells are still capable of launching an attack, experts say. "The optimist would say that al Qaeda has been effectively decapitated -- that the leadership at least is separated from its normal way of doing business with its followers," said Ellen Laipson, president of the Stimson Center, a Washington think tank specializing in security issues. "Even if we were more or less sure that bin Laden was disabled or no longer with us, we still believe that the organization probably had enough contingency plans that it could continue to function at some level," said Laipson, former vice chairman of the National Intelligence Council. MORE STRIKES Last December, the government of Singapore arrested 15 suspects linked to al Qaeda for allegedly plotting to bomb U.S. targets there. U.S. officials believe the network is planning more strikes. "What else are they living for?" one U.S. official said. "There is no peace negotiation involved ... It is for them a struggle to the death with their enemy." The U.S. government estimates that one-third of about two dozen senior al Qaeda leaders are either dead or captured. If bin Laden and Zawahri are dead, one prominent al Qaeda figure who might fill the gap is Khalid Shaikh Mohammed, who U.S. officials suspect of involvement in the Sept. 11 plot and a plan to blow up U.S. airliners over the Pacific in 1995. The threat from al Qaeda was unlikely to end soon. "It is one of these flatworms: you can chop it up and the pieces can still be alive and function, and then regenerate in various ways," a U.S. official said. "To make it go away completely is not something we're going to see any time soon."