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By Thursday some $342 million had been returned - still far short of the total, but enough to raise furious speculation over the motives of the hacker.
In messages embedded in the transactions, the thief insisted they stole with good intentions. "I am not very interested in money!" they wrote, adding it was "always the plan" to return the stolen funds.
Thieves typically try to cover their tracks by splitting the money up and moving it around - "sometimes using hundreds of thousands of consecutive transactions". The returns began after SlowMist, an investigative firm, claimed to have identified some of the hacker's personal details, including their email.
Some investors would also consider it a "fair bargain" for the hacker to keep some of the money, as a reward for finding the security flaw.
Cryptocurrencies like Bitcoin and Ethereum have soared in popularity in recent years. Their combined market value currently stands at nearly $2 trillion, creating alluring prospects for hackers.
Crimes involving cryptocurrencies are on a downward trend, despite spectacular thefts like this one and concerns about their use by criminal gangs. A report this month by security firm CipherTrace estimated global crypto-crime losses at $1.9 billion last year, down from $4.5 billion in 2019.
It did, however, warn of an alarming rise in hacking and fraud linked to decentralised finance, or "defi" - a form of crypto-financing, including loans, designed to cut out intermediaries like banks.
The Poly heist is part of that trend, with the company calling it the biggest hack "in defi history". "The imagination of fraudsters in this industry is constantly developing," said Syedur Rahman, a British lawyer who specialises in cases involving cryptocurrencies.
Tighter regulations are increasingly forcing cryptocurrency exchanges to verify users' identities, while law enforcement agencies are growing more experienced in handling crypto-crimes.
Hackers extracted a $4.4 million ransom in Bitcoin from oil company Colonial Pipeline in May, but the FBI was able to track down most of the coins and seize them.
With inputs from AFP
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