(CBS News) U.S. officials are increasingly concerned about Al Qaeda in the Arabian Peninsula and its master bomb maker, Ibrahim al Asiri, who allegedly built the underwear bomb used to nearly blow up an airliner over Detroit on Christmas Day 2009.
There are few specific details about the Qaeda terror threat that led the State Department this week Sunday and prompted U.S. officials.
However, Transportation Security Administration chief John Pistole has said that al Asiri developed a new type of explosive that might be able to slip through security.
"All of our explosive detection equipment, which screens over a million checked bags every day just in the U.S., wasn't calibrated to detect that," Pistole said.
Analysts believe somewhere in Yemen is the most likely target and ominously warn it could be anywhere in the arc of mostly Muslim countries.
(Watch CBS News correspondent Jeff Pegues report the latest on the threat at left)
"I don't think you'd see the U.S. government closing 22 facilities in 18 countries unless they were concerned that this was a real and significant plot from a serious terrorist group," CBS News senior national security analyst Juan Zarate said on "CBS This Morning: Saturday." "Every indication suggests that this is real plotting and real planning by al Qaeda, and the problem for U.S. officials is they do not have specificity with respect to where they might hit or the timing."
A travel alert alerted Americans to what officials call the "continued potential for terrorist attacks" with "public transportation systems and other tourist infrastructure" as potential targets.
"Obviously it's serious enough that we are taking concrete steps to make sure our personnel overseas are safe," House Intelligence Committee Chairman Mike Rogers, R-Mich., said.
Installations of other western countries are affected as well. Britain has closed its embassy in Yemen and is urging all its citizens to leave the country.
"If we aren't able to get to the bottom of the threat or to disrupt it definitively, the question for U.S. officials will be at what point does the travel alert, at what point do we re-open the doors of our embassies?" said Zarate. "We have to do diplomacy. We have to be present, and a key question is at what point is the threat no longer real? The challenge for intelligence officials is we may never know."
For Juan Zarate's full interview, watch the video in the player above.