Finland's customs agency has been struggling with what to do with a horde of bitcoin (BTC) it fears could end up back in the hands of criminals if sold off.

Finnish Customs, known locally as Tulli, has been trying to offload a total of 1,666 bitcoin for several years, even drawing up a plan in September 2018 for a public auction of the digital coins. But officials have concerns that a sale would attract the wrong kind of attention and could even put the agency's own security at risk.

Speaking to local media, Tulli Director Pekka Pylkkanen said: "From our point of view, the problems are specifically related to the risk of money laundering. The buyers of [cryptocurrency] rarely use them for normal endeavors."

Tulli confiscated the bitcoin trove following a successful bust of an online dark market in September 2016. At the time, bitcoin traded at around $570, meaning the 1,666 BTC was worth approximately $950,000. With prices now just under $9,200, it's worth closer to $15 million, according to CoinDesk's Bitcoin Price Index.

At bitcoin's all-time peak near $20,000 in December 2017, the cache would have been worth almost $33 million.

Tulli isn't the only government authority having to decide what to do with confiscated bitcoin, usually with dollar-values many times greater than when they were first seized. The U.S. government, which has seized hundreds of millions of dollars worth of bitcoin over the years, has hosted multiple online auctions for confiscated bitcoin.

Bitcoin confiscated by the Belgian authorities was sold by an online auction house in early 2019. Later that year, U.K. police used the same auctioneer to sell more than $290,000 worth of cryptocurrency it had seized from a teenage hacker.

In 2018, the Finnish government barred customs officials from trying to sell seized bitcoin on exchanges or trading platforms, instead ordering the agency to hold any confiscated digital assets in a secure cold storage solution.

Pylkkanen's claim that most crypto holders use them for illicit purposes isn't supported by the numbers. A November report from blockchain analytics firm Elliptic suggested $829 million in bitcoin, just 0.5 percent of all transactions, were linked to the dark web.