After the recent WannaCry and NotPetya attacks, hackers have taken ransomware mainstream. But another type of criminal is also demanding bitcoin in exchange for releasing something, or someone: traditional kidnappers.

Earlier this month, The Indian Express reported a kidnapping gang had attempted to solicit payment in bitcoin. In a sign of how technology and crime intersects, sometimes unsuccessfully, this is far from the only case of kidnappers trying to use cryptocurrency.

Six people abducted a trader called Ashu Jain on May 30, according to The Indian Express. The gang allegedly demanded 20 bitcoins, or around $52,000 at today's exchange rate. Deepak Sharma, an IT graduate allegedly led the scheme, the report adds.

Earlier this year, kidnappers targeted a Brazilian woman who was, notably, married to a bitcoin businessman. According to a translated statement published by Cryptocoins News, the officer in charge of the investigation said, "I spoke with a few colleagues from all over Brazil and there was never a kidnap attempt in which a payment in virtual currency was demanded. This is unprecedented in Brazil."

But the practice stretches back at least as far as 2015, when Taiwanese kidnappers pocketed over $1.5 million worth of cryptocurrency, according to Cointelegraph . The gang kept hold of Wong Yuk-kawn, the chairman of a Hong Kong-listed oil company, for 38 days. The same year, kidnappers in Costa Rica targeted a Canadian expat and demanded tens of thousands of dollars in bitcoin.

Bitcoin is often mistakenly portrayed as some totally anonymous form of currency that allows criminals to obtain funds largely undetected. That's just not always true—multiple alleged dark web dealers have been identified by their use of bitcoin. And as soon as the crime involves stepping out into the physical world, such as posting quantities of drugs in the mail, investigators have another avenue to pursue.

That may be the case with some kidnappers and bitcoin too: in all three of these cases, authorities arrested suspects. In the Indian example, police picked up six people, as did the Brazilians. In Taiwan, investigators arrested 16 people.

But as bitcoin continues to gain more attention, maybe more traditional criminals will try to merge their trade with cryptocurrency.