The terrorist attacks in Saudi Arabia (search) are a fresh reminder that months of hunting for Usama bin Laden have failed to flush out the Al Qaeda leader.

Asked at a Pentagon news conference Thursday why the United States cannot find bin Laden, Defense Secretary Donald H. Rumsfeld (search) turned to his senior military adviser, Gen. Richard Myers (search), and said, "That's a good question" -- putting the Joint Chiefs chairman on the spot for an explanation.

Myers said it appears that bin Laden and his top lieutenants are "in areas where they can bribe the [people] that are inclined to support them and protect them, and they are in very difficult areas on this Earth."

"That doesn't mean that we don't have people out this very minute out looking for him, as a matter of fact," he added.

Rumsfeld then added his own explanation.

"Why haven't we found him? One, it's very hard to find a single individual in the world. It's a big place." He said that when the bin Laden search escalated after the Sept. 11, 2001, terrorist attacks, the U.S. military was not organized to do manhunts.

Also, the terrorist leaders, who had used Afghanistan as a training and organizing base before the ruling Taliban was toppled, have the advantage of hiding in ungoverned areas. He mentioned none by name, but the tribal areas of western Pakistan, along the Afghan border, are one example.

"Third, there are still countries that are harboring terrorists," Rumsfeld said. "I mean, we know there are senior Al Qaeda in Iran, for example, presumably not an ungoverned area."

Rumsfeld's bottom line: The task is "difficult and challenging, but not impossible."

And his assessment of bin Laden's fate: "He's either alive, or he's alive and injured badly, or he's dead. Who knows?"