Authorities in Yemen released a female engineering student who had been brought in for questioning in an airline cargo bomb plot, as investigators around the globe strained to understand the outlines of the attempt and the masterminds behind it.
Only hours after White House counterterrorism adviser John Brennan said he was "confident" that a 22-year-old woman detained Saturday in Yemen had mailed the bomb-laden parcels, government officials in the country said they had released her, citing an apparent case of identity theft.
The confusion only served to underscore the increasing sophistication of terrorists using Yemen as a home base, and the still developing national security and law enforcement response in the U.S.
A team of federal investigators -- including FBI agents and transportation security officials -- is on the way to Yemen to hunt for clues in the case and advise about screening procedures, authorities confirmed. Among their first tasks will be to find out who stole the engineering student’s identity and mailed the packages.
"We are continuing to pursue aggressively a variety of investigative leads and are working very closely with Yemeni, British and Emirati officials to find those responsible for the attempted terrorist attacks," according to a U.S. government official.
The package bombs discovered last week — one in Britain, the other in Dubai — were both addressed to Jewish institutions in Chicago, but their ultimate target remains unclear.
"I believe the threat is certainly ongoing," Brennan said on the Sunday talk shows. "We just need to stay on top of this and work it diligently."
Brennan said authorities can't rule out that the two devices are the only ones out there. He added that investigators "have to presume" there might be more potential mail bombs.
FedEx and UPS have stopped importing packages from Yemen.
The two bombs were "very sophisticated" and "self-contained," Brennan added, apparently meaning they could have exploded without an external signal of some kind, "at a time of the terrorists' choosing."
The foiled plot "certainly bears the hallmark" of al-Qaida's Yemen branch, known as al-Qaida in the Arabian Peninsula, and the terrorist group is "still at war with us, and we are very much at war with them," Brennan said.
Forensic analysis indicates that the bomb-maker also constructed the devices used in the failed bombing on a Detroit-bound airliner last Christmas and the attack on Saudi Arabia's counterterrorism chief last year, Brennan said.
The person assembling these devices, he told ABC's This Week, is "clearly somebody who has a fair amount of training and experience, and we need to find him and we need to bring him to justice."
U.S. intelligence officials believe the suspected bomb-maker is a Saudi named Ibrahim Hassan al-Asiri.
British officials say they believe the bombers intended to bring down the airplanes, and Brennan told CBS' Face the Nation that the U.S. backs that view.
"At this point, we would agree with the British that it looks as though they were designed to detonate in flight," he said.
But Brennan added that they are still working to figure out whether the airplanes or the Chicago synagogues were intended to be the final stop for the bombs.
The bombs all contained the explosive PETN.
NPR's Tom Gjelten told Weekend Edition Sunday host Liane Hansen that the explosive powder was packed into toner cartridges, in order to hide it from scanning.
Meanwhile in Yemen, police released Hanan al-Samawi, whose detention had prompted protests among her fellow students at Sanaa University. They are now looking for a woman who may have impersonated her at UPS and FedEx offices in the country.
Gjelten said authorities apparently traced the woman through a phone number that she allegedly left.
"It suggests that the Yemeni authorities want to show that they are cooperating, but it's far from clear what responsibility this woman had," Gjelten said.
A Yemeni human rights group said al-Samawi is not known to be involved in any political activity or to have ties to any Islamic groups. After the apparent dead end, investigators were hunting the impoverished Mideast country for more conspirators. Authorities were also looking at two language institutions the plotters may have been associated with.
The explosives were pulled off airplanes in England and the United Arab Emirates early Friday morning, touching off a tense search for other devices. More details emerged Saturday about the plot that exploited security gaps in the worldwide shipping system, and prompted calls for tougher screening of air freight.
British Prime Minister David Cameron said he believes the explosive device found in central England was intended to detonate on the plane, while British Home Secretary Theresa May said the bomb was powerful enough to take down the aircraft. A U.S. official said the second device found in Dubai was thought to be similarly potent.
Brennan said the devices, which other officials report were wired to cell phones, timers and power supplies, did not require someone to physically detonate them while the planes were in the air, or when the packages were halfway around the world in the U.S.
Still, the fact that the devices made it onto airplanes showed that nearly a decade since the Sept. 11, 2001, attacks, terrorists continue to probe and find security vulnerabilities.
Qatar Airways released a statement Sunday saying the bomb discovered in Dubai was flown out of Yemen on one of its flights by way of Doha, the Qatari capital.
A spokesman for the airline, speaking on condition of anonymity in line with the company's standing policies on conversations with the media, said the parcel traveled on two separate passenger planes.
And The Associated Press, citing an unnamed U.S. official and a British security consultant, said investigators may have overlooked one of the devices the first time they swept a cargo plane at East Midlands airport.
After Dubai authorities reported finding a suspicious toner cartridge, the U.K. investigators searched again and unearthed a questionable device on a plane there, the AP report said.
In response to questions on the Sunday talk shows, Brennan said that investigators are giving a fresh look to the Sept. 3 crash of a UPS cargo plane in Dubai. But investigators in the United Arab Emirates said Sunday there was no evidence that an explosion caused that crash.
Two pilots were killed in the crash after a fire began in the cargo area and smoke filled the cockpit. Investigators initially expressed concern about a shipment of lithium batteries onboard.
NPR's Tom Gjelten contributed to this report, which includes material from The Associated Press.
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