United States officials are tying the new threat of shoe bombs on airplanes to al Qaeda.
There are indications that the threat could ultimately be tied to Ibrahim al Asiri, the master bomb-maker for al Qaeda in the Arabian Peninsula.
He's the man officials say designed the underwear bomb that failed to detonate aboard a Detroit bound flight on Christmas Day in 2009.
He also built the printer-cartridge bombs that al Qaeda placed on cargo planes destined for Chicago the following year.
Al Qaeda link to shoe-bomb warning
An official tells CNN the new warning is related to recent intelligence gathered on bomb-making tactics believed tied to Asiri.
CNN's Erin Burnett talks to National Security Analyst and Former CIA operative Bob Baer about the latest threat and why Asiri hasn't been caught.
"He's up in the mountains of Yemen," Baer says. "There's no digital footprint that he's left behind. He's read about leaks in this country. He's fairly immune and there's nothing the Yemeni government could do about it."