In a phone interview last week, Levine said those two stashes were found by analyzing a transaction Karpeles made in 2011 when Mt. Gox was pressured to prove the company was solvent.
At that time, Karpeles is believed to have moved just over 424,242 bitcoins between two Mt. Gox addresses. Since the transaction was recorded in the blockchain, it would ostensibly be proof that Mt. Gox had the bitcoins.
Levine, who wrote about their findings, cautioned though that their conclusion may not be accurate. There are a lack of technical tools to perform deep analysis of the blockchain that could make it easier to elicit more definitive conclusions, he said.
"There's a lot of technical depth, but when it comes to attributing it to individuals, it's very, very difficult, and it's tempting to draw conclusions because sometimes it seems like it's just obvious," he said.
The 850,000 bitcoins that were lost from Mt. Gox, 100,000 of which were its own, were worth an estimated $474 million. If stolen, the incident would be one of the largest cyber crime thefts on record.
An academic paper published last year that analyzed noted thefts of bitcoins found that following a trail of bitcoins was hard if a thief used certain techniques, including splitting balances into many other addresses, but few did.
"For the thieves who used the more complex strategies, we saw little opportunity to track the flow of bitcoins (or at least do so with any confidence that ownership was staying the same), but for the thieves that did not there seemed to be ample opportunity to track the stolen money directly to an exchange," they wrote.
Because bitcoin is just five years old, law enforcement may still be just catching up with how bitcoin works, let alone honing blockchain forensic techniques.
"A lot of people think of bitcoin as funny money," said Bruce Fenton, board member of The Bitcoin Association, a nonprofit industry organization. "This is serious money for serious people."
Another possible scenario that Mt. Gox simply lost the private keys to the bitcoins, which are required to transfer the virtual currency to another address, through a hardware failure or a software error.
If that's the case, it would appear by looking at the blockchain that Mt. Gox would still have bitcoins sitting in an address known to be under its control, but transferring the bitcoins is impossible.
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