NEW : Next terror attack on U.S. likely to originate in Pakistan tribal areas , report says . Comment: Biological attack more likely than nuclear attack , report says . Number of nations with nuclear weapons also growing , panel says .
NEW : Next terror attack on U.S. likely to originate in Pakistan tribal areas , report says . Terrorists with biological weapons could kill millions , panel 's chairman says . Biological attack more likely than nuclear attack , report says . Number of nations with nuclear weapons also growing , panel says .