HONG KONG - Cryptocurrency platform Poly Network said on Monday that almost all of the $610 million stolen this month in one of the biggest crypto heists had now been returned by the unknown person or persons behind the attack.

In a Twitter post, Poly Network said it had regained control of all the assets except for $33 million in stablecoin tether that had been frozen by the company that manages it. The network said it was in talks with tether about unfreezing those funds.

The hackers had previously said they did the attack for fun to expose a vulnerability in the platform's digital contracts and it was always their plan to return the tokens. Some blockchain analysts have speculated they just found it too difficult to launder so much stolen cryptocurrency.

A lesser-known name in the world of crypto, Poly Network is a decentralized finance (DeFi) platform that facilitates peer-to-peer transactions with a focus on allowing users to transfer or swap tokens across different blockchains.

Poly Network announced the hack on Aug. 10 but said the perpetrators had started returning the digital coins the following day. The network also offered the hacker or hackers a $500,000 "bug bounty".

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