WASHINGTON -- A U.S. official says the head of the top counterterrorism agency is resigning after nearly five years at his post, now that one of the nation's major counterterror goals has been met with the killing of Osama bin Laden last month.Michael Leiter presided over a reorganization of how counterterrorism information is shared, after the 2009 attempted bombing of a Detroit-bound airliner was blamed on a failure to share intelligence among government agencies.Read More... More on Osama bin Laden