PHILADELPHIA (October 13, 2012)--The FBI is investigating an unusual heist of some newly designed $100 bills that aren't going into circulation until next year.

Agent Frank Burton, Jr. said the cash was stolen from a plane that arrived at Philadelphia International Airport at around 10:25 a.m. Thursday from Dallas.

Investigators said the bills are easy to spot because they have sophisticated elements to thwart counterfeiters, such as a disappearing Liberty Bell in an orange inkwell and a bright blue security ribbon.

The FBI said a "large amount" of bills were stolen, but agents weren’t giving specifics.

The theft was reported by a courier service transporting the bills when the shipment arrived Thursday afternoon at the Federal Reserve Building in East Rutherford, N.J.

Officials then discovered some of the money was missing.