Unknown thieves stole a "large amount" of newly-designed $100 bills bound for a Federal Reserve facility in New Jersey on Thursday, the FBI said.

Frank Burton, Jr., spokesman for the FBI's Philadelphia division, said the theft occurred at some point between when the shipment of bills landed at the Philadelphia airport on a commercial flight from Dallas at 10:20 Thursday morning, and when the shipment reached its New Jersey destination around 2:00 p.m., when the courier service transporting the bills reported some missing.

Burton declined to comment on the amount taken, but said it was substantial.

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"We're following all logical leads," he said

The missing bills carry a design that is not slated to reach circulation until 2013. They feature a large gold "100" graphic on the back, and an orange box on the front with a faint image of the Liberty Bell.