The would-be bomber was supposed to buy a plane ticket to the United States and detonate the bomb inside the country, officials said.
Sen. Dianne Feinstein, D-Calif., who heads the Senate Intelligence Committee, told reporters Monday night that she had been briefed about an "undetectable" device that was going to be on a U.S.-bound airliner.
Before the bomber could choose his target or buy his ticket, however, the CIA moved in and seized the bomb.
The fate of the would-be bomber remains unclear. Rep. Peter King, R-N.Y., chairman of the House Homeland Security Committee, told CNN on Tuesday that White House officials told him, "He is no longer of concern," a point Brennan echoed on a round of appearances Tuesday on television news shows.
"We're confident that this device and any individual that might have been designed to use it are no longer a threat to the American people," Brennan said.
The plot was a reminder of the ambitions of al-Qaida in Yemen, the most active and dangerous branch of the terrorist group. While al-Qaida's core in Pakistan has been weakened over the past decade, instability in Yemen has allowed an offshoot group to thrive and set up training camps there. In some parts of the country, al-Qaida is even the de facto government.
Though analysis of the device is incomplete, U.S. security officials said they remained confident in the security systems that are in place.
"These layers include threat and vulnerability analysis, prescreening and screening of passengers, using the best available technology, random searches at airports, federal air marshal coverage and additional security measures both seen and unseen," Homeland Security spokesman Matthew Chandler said.
"The device did not appear to pose a threat to the public air service, but the plot itself indicates that these terrorists keep trying to devise more and more perverse and terrible ways to kill innocent people," Secretary of State Hillary Rodham Clinton said during a news conference in New Delhi with Indian External Affairs Minister S.M. Krishna.
It's not clear who built the bomb, but because of its sophistication and its similarity to the Christmas Day bomb, authorities suspect it was the work of master bomb maker Ibrahim Hassan al-Asiri or one of his students. 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.
Both of those bombs used a powerful industrial explosive. Both were nearly successful.
But the group has also suffered significant setbacks as the CIA and the U.S. military focus more on Yemen. On Sunday, Fahd al-Quso, a senior al-Qaida leader, was killed by a missile as he stepped out of his vehicle along with another operative in the southern Shabwa province of Yemen.
Al-Quso, 37, was on the FBI's most wanted list, with a $5 million reward for information leading to his capture. He was indicted in the U.S. for his role in the 2000 bombing of the USS Cole in the harbor of Aden, Yemen, in which 17 American sailors were killed and 39 injured.
Al-Quso was believed to have replaced Anwar al-Awlaki as the group's head of external operations. Al-Awlaki was killed in a U.S. airstrike last year.
The new Yemeni president, Abed Rabbo Mansour Hadi, has promised improved cooperation with the U.S. to combat the militants. On Saturday, he said the fight against al-Qaida was in its early stages. Hadi took over in February from longtime authoritarian leader Ali Abdullah Saleh.
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