Actually, the defense secretary doesn't know that for sure. And as he makes clear, he believes that Osama had help in hiding all those years in Pakistan.

The Hill:

U.S. officials have "no evidence" that current Pakistani leaders knew Osama bin Laden had been holed up near their capital city for half a decade, but they suspect someone - perhaps a retired official - did know about the al Qaeda leader's whereabouts.

Amid passionate calls from Capitol Hill for the Obama administration to get tough with Pakistan after the revelation bin Laden had been hiding there for up to five years, Defense Secretary Robert Gates said Wednesday he has seen "no evidence at all that the senior leadership knew."

Gates told reporters during a Pentagon briefing that "I've seen some evidence to the contrary."

In fact, Gates said, U.S. officials "have no evidence yet with respect to anybody else" knowing bin Laden was hiding in a compound about 30 miles from Islamabad.

Still, the former CIA director added, "My supposition is somebody knew."

Gates said a retired Pakistani official or a "low-level" one probably was in the know, reiterating his feeling was "pure supposition."

"It's hard to go to them with an accusation when we have no proof that anybody knew," Gates told reporters.

The idea that a "low level" official could have protected the terrorist from discovery all these years is not credible. At the very least, general officers in the Pakistani military and high level intelligence officials had to have been aware of Osama's presence, if not actively working to keep his location secret.

"Plausible deniability" isn't very plausible in this case.