U.S. Director of National Intelligence James Clapper testified before the Senate Armed Services Committee during hearings assessing threats to U.S. national security.
Clapper said it was unlikely that foreign terrorists groups would attack the United States using a chemical, biological, radiological or nuclear weapons within the next year because of what he said was "intense" counter-terrorism pressure.
Nevertheless, he said, the U.S. intelligence community was on high alert from the CBRN threat because of the spread of technology and the minimal infrastructure needed for some of those weapons.
Clapper stressed that while a "mass attack" was unlikely, groups like Yemen's al-Qaida in the Arabian Peninsula have expressed interest in acquiring the capability to carry out a limited attack using such weapons.
AQAP ideologue Anwar al-Awlaki was killed last year in Yemen in an attack by a CIA drone. He was said to have inspired several attacks targeting U.S. national interests, including a plan to down a plane in Detroit on Christmas Day 2009.
Nigerian Umar Farouk Abdulmutallab was sentenced to four life terms Thursday by a U.S. district judge in Detroit for his role in that attack.