New versions of $100 bills scheduled for circulation next year were stolen Thursday during or after a flight from Dallas to Philadelphia, the FBI said.

A law enforcement source said $20,000 was stolen from a total shipment of several million dollars.

The plane, which was not identified, arrived about 10:25 a.m. at Philadelphia International Airport, Special Agent Frank Burton Jr. said Friday.

A courier service transporting the money reported the theft about 2 p.m. when its employees arrived at the Federal Reserve Building in East Rutherford, N.J., Burton said.

The money container was opened and some of the money was missing, Burton said.

"We're looking at the airport employees as well as the courier," Burton said.

Burton would not say how much money was stolen other than to describe it as "a large amount."

Merchants who are offered the hot cash or anyone else with information about the heist should call the FBI at 215-418-4000, Burton said.

The new bills have distinctive orange coloring and a large "100" on the back.