If you have a deep, dark secret you want to keep to yourself, you may be vulnerable to a pricey scam.

Extortion letters are starting to circulate in Portland. They say, for a hefty fee -- your secret is safe!

“They are consistently looking for areas where people are vulnerable,” said Pamela Levy, whose husband received an extortion letter in the mail.

“The more information somebody has, the more effective it is," said Renn Cannon, a special agent with the FBI.

The details in these extortion letters are just "specific" enough, to make them believable. And at the same time, they're just "vague" enough to cover a range of secrets, from infidelity, to financial, to criminal. Of course, if you live a clean life, you've got nothing to worry about.

That was the case for Levy, happily married for years, but it’s not the case for everyone.

The letters start with a threat:

"I know about the secret you are keeping from your wife, and everyone else, more importantly, I have evidence of what you've been hiding. I'm going to give you two options, ignore this, and I’ll tell your wife, her friends and family. Or you can pay me."

Levy says she opened it and called her husband.

“My husband wasn't there with me at the time, and I called him up and said you're not going to believe what just came in the mail," she said

Luckily, Levy and her husband have been happily married for years but the FBI says these kind of extortion letter scams are becoming common.

“The more information they have about you the more it seems like ‘Well, boy they really know their stuff,’ so it does make it more effective," said Cannon.

The scammers use your name, and reference your city, just enough to make it convincing. The letter goes on to demand a "confidentially fee." Pay $3,600 dollars and your secret remains safe.

“It's the kind of threat that may ring true for a small selection of those where they may actually decide, I'm going to go ahead and send some cash," Cannon said.

"I knew instantly it was a scam, and I called my husband and he knew instantly it was a scam, and we both started laughing about it," Levy said. "But as a concerned citizen, I started to be really worried about all the people who might not know it was a scam."

Extortion is a federal crime, but catching someone is tough because the letter wants the payment in bitcoin, and even gives step-by-step instructions on how to pay the hush money. The FBI says that makes it hard to trace the scammers.

“Because its decentralized, because there's no central authority, because its new technology, because its encrypted; it makes it very hard to trace," said Cannon.

But the FBI, state and local investigators are successfully working these cases, especially when there's a lot of money lost. Levy says a sense of humor helps. She says she never once considered paying a ransom, even if her husband was up to no good.

“No, I considered jewelry. No bitcoin for anybody else," she joked.

For the scammers, it's a phishing scheme. They send out thousands of these letters across the country. Odds are some of the recipients will be up to no good, and will pay to keep their secrets safe.

If you get one of these letters, you can go to the FBI’s "internet crime complaint center" at www.ic3.gov. They also advise people to be careful about personal details they share online.