When passengers boarded Swiss International Air Lines Flight 17 from Zurich to New York City last weekend, they probably had no idea that the cargo hold beneath their feet was also functioning as a bank vault, carrying tens of millions of dollars.

A U.S. bank was sending cash from its Swiss office back home — something that occurs with surprising frequency, even in the digital age. While money can be wired, credited, debited and transferred around the world in a split second, there has never been more U.S. currency in circulation, and the amount keeps growing. And that cash moves around frequently, often carried discreetly in the belly of commercial airliners.

Someone, it seems, was wise to the shipment on the Swiss plane.

By the time the cash reached its destination, a Federal Reserve center in the New York area, more than $1 million had vanished.

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“We are investigating the apparent disappearance of $1.2 million in U.S. currency,” James Margolin, an FBI spokesman, said Tuesday.

Much about the theft remained unknown, including exactly where and when the money disappeared.

The last time the money was seen was before it was loaded onto the plane in Zurich, according to a law-enforcement official who insisted on anonymity because the investigation was continuing.

The cash that was stolen — 12 packets of $100 bills, each totaling $100,000 — had been placed in a secure container that then was placed inside a larger container, the official said.

The flight arrived at Kennedy International Airport at 2 p.m. Saturday. The container was unloaded and sent to the Federal Reserve center. Only when it arrived there did the authorities discover that the money was missing.

“What we don’t know is at what point in the journey that the money disappears,” the law-enforcement official said.

An airline official, speaking on the condition of anonymity, said there was no indication that the money had been stolen from the plane. However, the U.S. law-enforcement official said it was too early to reach any firm conclusions.

If the money was stolen at Kennedy Airport, it would rank as one of the largest robberies in the airport’s history — in 1978 an estimated $5 million in cash and $1 million in jewels were stolen from a Lufthansa Airlines cargo building, a crime that was dramatized in the movie “Goodfellas.”

The news of the theft was reported Tuesday by The New York Post.

Susanne Muhlemann, a spokeswoman for Swiss International Air Lines, declined to comment on the theft.