The FBI has issued a warning on pet adoption scams along with ways to avoid getting duped.

Earlier this week, the Portland Oregon office of the FBI offered tips on “building a digital defense” against pet adoption scams.

“Fraudsters are prowling the web to find families who are so desperate for the perfect pooch that they are willing to adopt sight unseen,” the FBI’s Beth Anne Steele wrote.

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The scam usually goes something like this: someone comes across a pet adoption ad online then unwittingly engages with the scammer and proceeds to pay a series of fees. But the pet never arrives.

A more detailed description is offered by Fraud.org, which is operated by the National Consumers League, a long-established American nonprofit consumer organization.

Once you begin sending money, the scammer then asks for additional funds to cover the cost of things like a special shipping crate or insurance or a host of other pretexts, Fraud.org says.

“Regardless of how much money is sent, the alleged seller will find new reasons to ask for additional payment. This continues until the victim, now often out hundreds or thousands of dollars, catches on and stops sending money,” Fraud.org says.

The organization posted a complaint from a woman who ended up losing hundreds of dollars before she realized it was a scam.

Often scammers ask for funds to be sent via Western Union or prepaid cards.

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Fraud.org cites popular online ad sites such as Craigslist and Oodle as the starting point for scams.

The International Pet and Animal Transportation Association (IPATA) says these scams can also originate in local newspapers, email, and pet travel sites. And the scammers sometimes pretend to be faith-based organizations. IPATA provides a regularly updated list of scammers.

The FBI offers the following advice:

Do your best to see the pet in person first.

Don’t pay for shipment if you can’t verify that the seller is reputable.

Do your homework by checking credentials and reading reviews from prior customers.

If you virtually chat with the seller, watch for broken English and odd phrasing and typos.

If the seller asks you to pay via wire transfer or gift card, don’t -- that's a tip-off it's a scam.

If you have been scammed, you can file an online report at the FBI’s Internet Crime Complaint Center at www.ic3.gov or call your FBI local office.