The White House says a plot to ship powerful, hidden explosive devices to the United States from Yemen is an ongoing threat, and that more such devices may exist beyond the two that were discovered in Britain and the United Arab Emirates.
President Barack Obama's counter-terrorism advisor, John Brennan, says a massive effort is underway to locate any additional explosive materials that may exist in the international air cargo system.
"Only two have been found," Brennan said. "We cannot presume, though, that there are none other that are out there. And so we are looking at all the packages that originated in Yemen. We have been able to identify all of them, and we are now putting them through some very thorough screening. We have suspended those cargo shipments to the Unite States that originate in Yemen."
Brennan was speaking on Fox News Sunday.
He described the bombs, hidden in ink cartridges for photocopiers, as sophisticated devices. Although the packages were addressed to Jewish houses of worship in Chicago, Brennan said the explosives could have detonated before reaching their destination.
"It [the bombs] did not require any additional components [in order to be detonated]," Brennan said. "It did not require somebody to go in and manually press a syringe or something else. This is something that could have been detonated en route to the United States."
Yemeni authorities have detained two women suspected of involvement in the plot.
Brennan says there is reason to suspect the involvement of a branch of al-Qaida that is active in Yemen, and that there are many questions that have yet to be answered.
"What we have to do is continue to investigate this, and see what else might be out there," Brennan said. "Are there other IEDs [improvised explosive devices] like this? Is this part of a broader effort? Al-Qaida in the Arabian Peninsula has been quite vocal in their threats against us, and there are individuals there who are very dangerous."
Terrorism experts believe the United States and its allies have made it much more difficult for al-Qaida to pull off massive operations like the September 11, 2001 attacks in New York and Washington. As a result, they say, terrorist groups are searching for ways to mount smaller-scale operations and exploit weaknesses that may exist in international security procedures.