New threat 'frightening,' Holder says

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Intelligence suggesting that bomb-makers from Yemen and terrorists from Syria are working together to create undetectable explosive devices is an "extreme" administration security concern, Attorney General Eric HolderEric Himpton HolderAmash: Trump incorrect in claiming Congress didn't subpoena Obama officials We can't allow presidents and public opinion to further diminish the work of the press Democrats sue over North Carolina's congressional maps MORE said Sunday.

“In some ways, it's more frightening than anything I think I've seen as attorney general,” Holder told ABC’s “This Week.”

Holder said the collaborative effort was a “deadly combination” of those with technical skills and "people who have this kind of fervor to give their lives in support of a cause that is directed at the United States and directed at its allies."

"It's something that gives us really extreme, extreme concern," Holder said.

Earlier this month, the Department of Homeland Security announced it was beefing up security measures at international airports out of concern over the terror threat. Individuals will now be required to turn on electronic devices such as cellphones and laptops at security checkpoints.

Holder said those measures were "prudent steps that are necessary to protect the flying public.”

"This is not a test," Holder said. "We're doing something in reaction to things that we have detected."

At the same time, the attorney general said he was "really confident in our capacity to understand what's going on, to monitor what's going on and working with our allies."

Earlier this week, Holder held a series of meetings in Oslo, Norway and London in a bid to encourage European allies to do more to counter the possible flow of terrorists into their countries.

He said the meetings were a chance to "exchange information in the way that you can't if you're not present" and "come up with techniques to thwart those who would try to do us all harm.”