LINCOLNWOOD, Ill. (CBS) — Police are investigating a pattern of mail theft and check fraud in north suburban Lincolnwood.

There are more than a dozen cases of people who say a fraudster tampered with their checks after they mailed them. CBS 2’s Tim McNicholas has learned it has also caught the attention of federal investigators.

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Jeff Zimmerman doesn’t know a Natalie Martinez. So imagine his surprise when he learned a check written out to her drained $4,000 from his bank account.

“I decided to go downstairs, talk to my wife, and tell her we’ve got a problem,” Zimmerman said.

Zimmerman soon realized that was a check he wrote out—not to Natalie Martinez, but to AT&T for a $125 bill. After he mailed it, someone pasted over that and added a new payee name and a new dollar amount.

In examining the check, one can still see Zimmerman’s own writing poking out behind the fraudster’s, which he showed to an employee at his bank.

“He looked at it and said to me, ‘So far this year you’re about the fourth or fifth person to come to me – the same problem,” Zimmerman said.

Zimmerman said he dropped the check in a blue mailbox in Lincolnwood back in August.

The Lincolnwood Police Department has taken 17 other reports over the past year, all from people who had their checks altered after mailing them. The department said it is working with U.S. postal inspectors to investigate the pattern.

“It’s terrible, just to think about it,” Zimmerman said.

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Earlier this fall, CBS 2 uncovered a pattern of mail theft and check fraud in another northern suburb – Park Ridge.

Court records say a Dolton man somehow got hold of a postal key and used it to steal from a blue mailbox outside the Park Ridge Post Office. He was arrested in September, but police said they are still investigating.

“It is a service that everyone depends upon,” said Kevin Kosar.

Kosar has studied the Postal Service for more than a decade with the American Enterprise Institute, a conservative think tank.

“It raises, in my mind, a question of, why is Chicago as a whole having so many issues with its mail?” Kosar said.

The U.S. Postal Inspection Service will not say whether there is any connection between the Park Ridge case and the one in Lincolnwood.

A spokesperson said postal inspectors are actively investigating mail theft in the Chicago area.

“It’s never happened to me before,” Zimmerman said. “You better watch where you’re mailing your bills, because this is happening all over now.”

When it comes to mailing bills, Zimmerman said he is not using the blue boxes in his neighborhood anymore.

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Postal inspectors won’t go into specifics, but they did say they’re investigating mail theft in the Chicago area. As for Zimmerman, he said his bank is investigating – but for now, he is still out $4,000.

Tim McNicholas