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(CNN) - The U.S. government has warned airlines to pay particular attention to the possibility of terrorists attempting to hide explosives in shoes, a result of new intelligence, according to two people familiar with the situation.
The acting CIA director said Thursday that he is specifically worried about this threat, as terrorists have figured out a way around the screening.
"Asiri's definitely getting better," said CNN national security analyst Bob Baer, a former CIA operative.
Ibrahim al-Asiri is believed to be the top bomb-maker for al-Qaeda in the Arabian peninsula.
The first shoe bomb attempts were "amateurish," says Baer, and missing key parts like detonators.
"These things can be made adequately on board, what they need is practice and they seem to be getting that," said Baer.
"They are using a particularly effective explosive ... and if placed right in an airplane, it could do major damage, at least kill people, if not cut a hole in the skin," said Baer.
For more of our interview with former CIA operative Bob Baer, check out the video above.