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DEL CITY, Okla. - It happens all the time.

Thieves targeting packages and letters from porches and mailboxes.

However, Theresa Manful thought this was almost unimaginable.

"From the post office? I don't know," she said.

Manful had gone to a Del City post office branch to buy a stamp and mail a $750 check to her aunt in Delaware.

"The clerk gave me the stamp for it, and he turned around and put it in the bin. So, I didn't even get to mail it. I just bought a stamp, and he did the rest of it. There's the mystery. We want to know how it got out of the post office," she said.

Aware of mail theft, Manful felt dropping it off at the post office was her best option.

"Safest way to do it, buy a stamp and have him do the rest. That's their job," she said.

However, that envelope never made it to Delaware.

Instead, the check somehow traveled 20 minutes away to a Bank First, where a mystery woman tried to cash it.

An alert bank teller became suspicious, and the woman ran out.

"He looked through the bank statements and realized we weren't writing that many checks. That was kind of a red flag for him," she said.

A theft report has been filed with the United States Postal Service.

Manful just wants the woman caught before she tries this again.

"We really need to find out who this mystery woman is and how it got out of the post office. That's what we really want to know," she said.

NewsChannel 4 did check with the Del City post office and showed them a picture of the mystery woman from the bank.

They said she does not work there.

We also received the following statement from the USPS.

"The Postal Service takes the sanctity and the security of the mail very seriously. This matter has been referred to the inspection service for further investigation."

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