There are growing concerns that an al Qaeda affiliate could use a new generation of liquid explosive, currently undetectable, in a potential attack, according to two senior U.S. government officials briefed on the terror threat that has prompted the closing of nearly two dozen U.S. embassies.
Though the Transportation Security Administration has long been concerned about liquid explosives being used in potential devices -- as it was during the failed Christmas Day bombing in 2009 -- the new tactic allows terrorists to dip ordinary clothing into the liquid to make the clothes themselves into explosives once dry.
"It's ingenious," one of the officials said.
Another senior official said that the tactic would not be detected by current security measures.
The officials said the new technique is believed to have been developed by the Yemen-based al Qaeda affiliate al Qaeda in the Arabian Peninsula (AQAP), home to notorious alleged bomb maker Ibrahim al-Asiri. Al-Asiri is suspected of being the mastermind behind several devious explosive devices including the underwear bomb and surgically implanted body bombs.
Al-Asiri was listed today among Yemen's 25 top terrorists, who the Yemeni government said were planning to carry out operations in the capital, Sana'a. The Yemeni government is offering 5 million Yemeni rials, or $23,000, for information leading to the capture of any of the terrorists.
Last month Transportation Security Administration chief John Pistole revealed details about a new and improved version of the underwear bomb, also thought to be al-Asiri's work, that he said would "possibly" have been discovered by TSA screening. That bomb was given to a double-agent last year, who gave it to western intelligence services.
The TSA declined to comment specifically on the new liquid device, but an official there said, "As always, our security posture, which at all times includes a number of measures both seen and unseen, will continue to respond appropriately to protect the American people from an ever evolving threat picture."
"I am not in a position to discuss any intelligence around this current threat. But, as a general matter TSA screens both passengers and carry-on baggage for metallic and non-metallic prohibited items, including weapons and explosives. To do this, TSA uses the best available imaging technology to safely screen passengers for any concealed items," the TSA official said.
Nearly two dozen U.S. embassies throughout North Africa and the Middle East were closed Sunday after the U.S. intercepted communications between the leadership of AQAP and al Qaeda's remaining leadership in Pakistan, which suggested a major operation was underway, senior U.S. officials said. The diplomatic posts are expected to remain closed this week.
In addition to the new liquid bomb, a U.S. official said American spy agencies are concerned the attack could use what some call "Frankenbombers," suicide bombers who could carry an improvised explosive device sewn into their body cavity.
Sunday on "This Week," Rep. Dutch Ruppersberger, D-MD — the ranking member of the House Intelligence Committee — said the intercepted communications called for a "major attack."
"It's a very credible threat and it's based on intelligence," Ruppersberger said. "What we have to do now is the most important issue, is protect Americans throughout the world."
Along with the embassy closings, the U.S. government is taking precautions by ramping up the use of federal air marshals on U.S.-bound flights.
Air cargo coming from Europe is under even more scrutiny by security services, one of the senior officials told ABC News, adding that intelligence analysts' best guess at the moment is an attack being planned against U.S. targets in Yemen or Pakistan. Both countries host U.S. Special Operations, counter-terrorism and intelligence officers targeting Al Qaeda with drones and direct action.
A Federal Aviation Administration notice issued last week also warned about a "significant risk to civil flight operations in Yemen" from terrorists armed with man-portable surface-to-air missiles.
Terror Threat, Mass Jailbreaks: An Al Qaeda Comeback?
The multiple embassy closings has thrust al Qaeda back into headlines around the world, shortly after the group garnered international attention for allegedly taking part in a series of mass jailbreaks last month.
Friday the international police organization INTERPOL released a "global security alert advising increased vigilance for terrorist activity" following the jailbreaks. INTERPOL said that al Qaeda was suspected to be involved in several of the plots and asked its member countries to help determine if they were coordinated or linked.
Following the May 2011 death of al Qaeda leader Osama bin Laden, top U.S. officials said that the core leadership of al Qaeda appeared to be on the ropes and, to quote President Obama, "on the path to defeat."
"Their remaining operatives spend more time thinking about their own safety than plotting against us," he said in a speech this May.
But Obama noted the "emergence of various al Qaeda affiliates," including AQAP which was the "most active in plotting against our homeland."
Nada Bakos, a former CIA analyst who was part of the team that hunted Osama bin Laden for years, told ABC News that any suggestion al Qaeda as a whole was down and out and is now seeing a resurgence is wrong. Instead, she said the group has just been undergoing a metamorphosis.
"An ideology has tentacles. That's why it's hard to predict how or if it will grow," she said. "Each of these groups [al Qaeda affiliates] are funded and operate independently, but they all share the same ideological platform that central al Qeada has propagated since the 1990s."
Seth Jones, associate director of the International Security and Defense Policy Center at the RAND Corporation, testified before Congress last month that despite the "weakness" of central al Qaeda, "there has been a net expansion in the number and geographic scope of al Qaeda's affiliates and allies over the past decade, indicating that al Qaeda and its brand are far from defeated."
Michael Scheuer, a former CIA officer who led the hunt for bin Laden before his retirement in 2004, went further, telling ABC News that he believed al Qaeda really hasn't changed since bin Laden's death.
"I think the guys on the ground [local affiliate commanders], day-to-day tactical decisions were made there, where they always have been," he said. "Core al Qaeda lays down the guidelines to keep everybody pointed at the enemy."
Scheuer said he believes al Qaeda is more dangerous under its currently leader, Ayman al-Zawahiri, than it was under bin Laden. Zawahiri, Scheuer said, may settle for smaller U.S.-based operations that would have a much smaller body count than a 9/11-type operation that bin Laden aspired to repeat.
Any idea that al Qaeda is on the way out, Scheuer said, is "completely politicking."
ABC News' Lee Ferran contributed to this report.