Unfortunately, there are a number of scammers who target elderly, retired people — and these scams can get very costly. Such was the case with a recent scam that occurred in Peachtree City, Georgia, during which an elderly couple was scammed out of $835,000 over a three-month period, Fox 5 Atlanta reported.

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The scam began with a pop-up on the retiree’s laptop computer screen stating that he had been hacked and instructing him to call a phone number immediately. Upon calling, the scammer told the man that his bank account had been hacked and that the money was being used for illegal activities. They were told that to prevent arrest, they needed to do such things as wire money and make Bitcoin ATM transactions. Once the couple realized they had been scammed, they had lost over $800,000.

Here’s a look at the red flags they missed.

Pop-Up Alerts That Include Phone Numbers

The first red flag the couple missed was the initial contact from the scammer.

“If you get [a] pop-up window on your computer, don’t call the number,” the Federal Trade Commission (FTC) states on its site. “Real security warnings and messages will never ask you to call a phone number.”

Instead, the best course of action is to turn off the computer and avoid interacting with these kinds of pop-up messages.

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A Caller With a Questionable Background

Scammers work to build trust with the people they are scamming. In this case, the scammer convinced the couple that they needed to provide the transfers of money in order to prevent them from being arrested, likely by impersonating a government employee.

In these cases, the FTC recommends taking steps to confirm the caller’s identity: “Stop. Check it out — before you wire money to anyone. Call the person, the government agency or someone else you trust. Get the real story. Then decide what to do. No government agency will ever ask you to wire money.”

Demands for Payment in Bitcoin

One of the ways the couple paid the scammer was via Bitcoin, likely at the specific request of the scammer. The FTC warns that if someone is demanding payment in the form of cryptocurrency, this is a red flag.

“Only scammers demand payment in cryptocurrency,” the FTC states on its site. “No legitimate business or government agency is going to demand you pay with cryptocurrency — not to buy something, pay taxes or fines, and not to ‘protect’ your money. That’s always a scam.”

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This article originally appeared on GOBankingRates.com: A Retirement Scam Duped a Senior Couple Out of $835K: 3 Red Flags They Missed