The United States has vowed to hunt down and kill new Al Qaeda boss Ayman al-Zawahiri. But much like his predecessor Osama bin Laden, al-Zawahiri has been in hiding since the September 11 attacks that he helped mastermind.

As we did both seek to capture and kill - and succeed in killing - bin Laden, we certainly will do the same thing with Zawahiri, US Admiral Mike Mullen told journalists Thursday.

But the government needs to find him first. His whereabouts are unknown, although Egyptian-born al-Zawahiri was long thought to be hiding on the tumultuous Afghanistan-Pakistan border. He is likely on the move, and he is now thought to be in Pakistan, whose government is being scrutinized for allowing terrorists such as bin Laden and Fazle-ur-Rahman Khalil to live unmolested on its soil.

It's difficult to guess where he would be right now. He might give us a surprise like bin Laden, who was living in a big villa in Abbottabad when we were expecting him (to be) somewhere in a cave in the mountains, a Pakistani intelligence official told reportedly told German Press Agency dpa on Thursday.

But despite his current illusiveness, the United States may be gathering positive leads as to the Al Qaeda chief's location. The Washington Post reported that some of the confiscated documents discovered in bin Laden's Abbottabad compound could help US forces locate al-Zawahiri's hiding spot.