Defense Secretary Leon Panetta and and China's Minister of National Defense Gen. Liang Guanglie, not seen, speak during a joint press briefing at the Pentagon, Monday, May 7, 2012. (AP Photo/Manuel Balce Ceneta) In this Sept. 7, 2011 file photo, John Brennan, Assistant to the President for Homeland Security and Counterterrorism, speaks in Washington. U.S. bomb experts are picking apart a sophisticated new al-Qaida improvised explosive device, Brennan said Tuesday. (AP Photo/Susan Walsh, File) This undated file photo purports to show Ibrahim Hassan al-Asiri. Al-Asiri constructed the first underwear bomb and two others that al-Qaida built into printer cartridges and shipped to the U.S. on cargo planes in 2010. U.S. bomb experts are picking apart a sophisticated new al-Qaida improvised explosive device, a top Obama administration counterterrorism official said Tuesday, to determine if it could have slipped past airport security and taken down a commercial airplane. (AP Photo/Saudi Arabia Ministry of Interior, File) Defense Secretary Leon Panetta speaks during a news conference with China's Minister of National Defense Gen. Liang Guanglie, not seen, at the Pentagon, Monday, May 7, 2012, in Washington. (AP Photo/Manuel Balce Ceneta)