The founders of South Africa’s largest cryptocurrency exchange have vanished, along with nearly $3.6 billion in Bitcoin, after telling investors the exchange had been hacked.

A Cape Town law firm, hired by affected investors, says it cannot find Ameer and Raees Cajee, the founders of Africrypt. The firm has warned crypto exchanges around the world to be on the lookout should anyone try to sell the digital currency on their platforms.

Africrypt warned clients of a hack in April, when Bitcoin was at its highs, according to Bloomberg. Oddly, though, it asked clients not to alert law enforcement authorities or lawyers as it “would slow down” the recovery process.

Not everyone bought that, and some investors retained Hanekom Attorneys to look into things. That firm found the company’s pooled funds had been transferred so many times and through so many filters that it was incredibly challenging—though not, technically, impossible—to trace the cryptocurrency.

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This would go down as the largest cryptocurrency loss in history if the money can’t be recovered. A report in April found that the 80 largest cyber thefts in the past decade resulted in collective losses of $2.5 billion.

All totaled, about 69,000 Bitcoins are missing from the firm. While they were worth $3.6 billion at the time of their disappearance, with recent drops, the value of that wallet currently stands at just under $2.4 billion.

Recovering the crypto could be complicated by the fact that South Africa’s Finance Sector Conduct Authority, which would normally handle these matters, is unable to start a formal investigation, since cryptocurrency isn’t legally recognized as a financial product in the country.

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This story was originally featured on Fortune.com