Some Silicon Valley residents have received a mysterious letter threatening to expose some deep, dark secret unless they’re paid a small fortune in Bitcoin. It’s an old con—preying on wealthy folks with a guilty conscience—but with a digital twist.

The Santa Clara County District Attorney’s Office sent out a public service announcement earlier this week to warn people about the scam, which has apparently targeted affluent communities throughout the nation.

It’s a fraud predicated on the letters ending up in the hands of people who have something to hide from their wives and access to thousands of dollars in hush money. The allegations are deliberately vague, but hint at infidelity.

“I’m going to cut to the chase,” one of the letters reads, according to the DA. “I know about the secret you are keeping from your wife and everyone else …”

The DA’s Office says the scammers probably know nothing about the would-be victims, but hope that they’ll snag a few by casting a wide enough net in a rich enough area. Even is someone bites, though, he or she is not necessarily off the hook.

“Once a victim falls for the scam and makes a payment, the blackmail seldom stops,” the DA cautions in a notice emailed to reporters on Tuesday. “The first payment will likely be followed by more—and increasing—demands.”

Since Bitcoin, like other cryptocurrencies, is untraceable, unaffiliated with any bank or government and can be cashed out anywhere in the world, the victim’s money ends up beyond the reach of U.S. law enforcement.

The DA’s advice for anyone who receives one of these letters: Ignore it. Or, for those who already fell for the trick, call the U.S. Postal Inspector’s Service at 877.876.2455.