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 report says . Biological attack more likely than nuclear attack , report says . Watch how officials worry about a biological terror attack » . Number of terrorists with nuclear weapons also growing , report 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 report says . Biological attack more likely than nuclear attack , report says . Number of terrorists with nuclear weapons also growing , report says .