Ayman al-Zawahiri, who took over Osama bin Laden as the leader of Al Qaeda, is somewhere in Pakistan, a former CIA official said.
Bruce Hoffman, a former CIA official who is a professor at Georgetown University, said that Zawahiri may be hiding in a populated area, just as Osama did.
He said that drones, which have proven so effective elsewhere in Pakistan, are 'problematic to use in built-up urban areas'.
The comments from Hoffman came as al-Qaeda confirmed a U.S. drone strike killed another top terrorist, Saudi Said al-Shahri, the co-founder of the group's Arabian affiliate.
Further, Thomas Joscelyn, a senior fellow with the Foundation for Defense of Democracy, said that targeting Zawahiri the similar way as the raid that killed Osama was unlikely following the high risk and local political consequences of the Navy SEAL raid.
According to the New York Daily News, with seasoned terror leaders now extremely cautious about using cell phones and other forms of electronic communication that can be tracked, the search for al Qaeda's remaining leaders relies heavily on human intelligence.
In the hunt for Zawahiri, the U.S. may be a victim of its own success.