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Increased security measures for some European and Middle Eastern airports with direct flights to the United States,聽ordered by the US Wednesday,聽reflect heightened concerns about the talents of one particular terrorist 鈥� and the contacts he may have made with a particular subset of jihadist terrorists.

The individual the US is worried about is Ibrahim Hassan al-Asiri, a Saudi-born Al Qaeda follower in Yemen whose particular talent is building hard-to-detect bombs 鈥� or perhaps even undetectable bombs.

The information that has the US ramping up security around US-bound flights is that Mr. Asiri may have established a working relationship with Jabhat al-Nusra, an Al Qaeda affiliate operating in Syria, according to some US counterterrorism officials.

Asiri is known for having equipped the 鈥渦nderwear bomber鈥� with the undetected explosives he attempted to detonate on a Christmas Day flight approaching Detroit in 2009.

Jabhat al-Nusra is known to have welcomed hundreds of Westerners, and in particular Western European Muslims, into its ranks. That鈥檚 the subset of Al Qaeda-linked terrorists the US is increasingly worried about.

The combination of Asiri鈥檚 bombs and terrorists with Western European passports boarding flights to the US could be devastating, American counterterrorism officials and experts say.

On Wednesday, Secretary of Homeland Security Jeh Johnson issued a statement saying his agency had shared 鈥渞ecent and relevant鈥� information with overseas partners. On the basis of that information, he added, he also directed the Transportation Security Administration to put new security measures in place in some airports because of 鈥渁n evolving environment.鈥�

In Britain, airport security officials said the new measures could include enhanced efforts to scrutinize electronics and shoes, in particular, for nonmetallic explosives.

But British government officials also hinted that the measures were not in response to a specific terrorist threat anticipated over the next few days, but that instead they reflected a new kind of threat that will affect airport security indefinitely.

鈥淚 don鈥檛 think we should expect this to be a one-off temporary thing,鈥� Britain's聽deputy prime minister, Nick Clegg, told London鈥檚 LBC Radio. He suggested that travelers should get used to the new measures.

What has counterterrorism officials so worried is that enhanced airport security measures may not be keeping up with Al Qaeda鈥檚 advancing technical skills 鈥� and in particular with Asiri鈥檚 quest to develop a virtually undetectable bomb.

Asiri, who is affiliated with Al Qaeda in the Arabian Peninsula (AQAP) in Yemen, is believed to be behind efforts to build nonmetallic bombs that could be sewn into clothing (such as the underwear bomb) or implanted directly into a suicide bomber鈥檚 body.

In one particularly chilling example of this technique, Asiri鈥檚 23-year-old brother Abdullah tried in 2009 to assassinate Saudi intelligence chief Prince Muhammad bin Nayef by detonating a bomb that had been inserted into his body. The bomb exploded prematurely, and only Abdullah al-Asiri died in the attack.

Booby-trapping human bodies with explosives is nothing new, and Westerners have become accustomed to hearing of suicide bombers wearing explosive vests carrying out attacks, especially in countries battling Islamist extremists. But intelligence suggests that terrorists are working to develop bombs that would be implanted surgically inside a suicide bomber and be undetectable to current security scanning systems, counterterrorism experts say.

One challenge that may be slowing someone like Asiri, they add, is developing an implantable bomb small enough to go undetected and to allow the would-be bomber carrying it to continue functioning, while at the same time delivering enough of a blast to destroy more than just the body of the bomber.

US and other Western counterterrorism officials aren鈥檛 certain how far Asiri and other bombmakers have come in their quest for an airliner-destroying body bomb. But they say that the threat posed by Westerners who have gone to fight in the jihadist trenches of Syria and Iraq is real today and isn鈥檛 dependent on perfection of an undetectable bomb.

It鈥檚 a threat that President Obama 鈥� perhaps in something of a precursor to this week鈥檚 enhanced airport security measures 鈥� spoke of in an interview with ABC鈥檚 鈥淭his Week鈥� last Sunday.

鈥淲e鈥檝e seen Europeans who are sympathetic to [the Islamist extremists鈥橾 cause traveling into Syria and now may travel into Iraq, getting battle-hardened. Then they come back,鈥� Mr. Obama said. 鈥淭hey鈥檝e got European passports. They don鈥檛 need a visa to get into the United States.鈥�