The counterfeit bills did not appear intended to fool customers so much as to trick the A.T.M.’s into believing they were carrying a full complement of cash. However, in most instances, the machines appeared to have been able to distinguish the fake bills from the real ones, and separate them. A Chase bank official said that the canisters designed to snare bogus bills for this purpose in the A.T.M.’s were full of them.

But at least two of the bills got through on Monday.

A customer at each of the two Chase branches alerted bank employees that they had received a fake bill, the bank official said. In one case, the customer had made a withdrawal for $20, while the other customer’s withdrawal was for $100, the official said.

Both customers discovered the fake bill right away, the official said.

In a statement issued by JPMorgan Chase & Company, the company said: “We are working to get all the facts and don’t want to come to any conclusions too early. Obviously, all of our customers who withdrew money will be made whole.”

It was not immediately clear whether any other A.T.M.’s had dispensed counterfeit bills.

An A.T.M. technician at a Chase bank branch on West 57th Street who inspected the counterfeits described the notes as “amateurish.” The technician said that the bills were cut correctly, but that visually it was easy to tell they were different.