The blockchain analytics firm Arkham Intelligence revealed on Saturday what may be the largest known crypto heist in history. The firm explained that the mining pool LuBian lost 127,426 Bitcoin in December 2020 after hackers breached its systems. The incident remained secret for nearly five years.

According to Arkham Intelligence’s post, LuBian controlled almost 6% of the Bitcoin network’s total hash rate around May 2020. However, on December 28th, the pool experienced a major breach that resulted in the loss of nearly 90% of its Bitcoin holdings. The following day, another $6 million worth of Bitcoin was stolen. On December 31st, LuBian moved the remaining funds into recovery wallets. Arkham Intelligence has since provided further details about the case.

“LuBian was a Chinese mining pool with facilities in China and Iran,” wrote Arkham Intelligence. “Based on analysis of on-chain data, it appears that 127,426 BTC was stolen from LuBian in December 2020, worth $3.5 billion at the time and now worth approximately $14.5 billion.”

Researchers believe the attackers used a brute-force strategy to compromise LuBian’s wallets. At the time, LuBian was reportedly using an algorithm to generate private keys that the hackers were able to exploit. Arkham also noted that several OP_RETURN messages — a type of blockchain transaction used to embed data — were sent from LuBian to the hackers’ addresses, pleading for the return of the stolen funds. The funds were never returned.

Despite the losses, LuBian managed to secure 11,886 BTC, now worth around $1.35 billion. The hackers, meanwhile, still control the bulk of the 127,426 BTC stolen. Arkham notes that both LuBian and the attackers appear to retain control of their respective assets. However, neither party has publicly confirmed the hack or commented on Arkham’s findings.

“With $3.5B in stolen assets at time of transfer, the LuBian hack is the largest ever,” wrote Arkham. “Owing to BTC’s price appreciation since 2020, the current value of stolen assets is $14.5B, making the LuBian hacker the 13th largest BTC holder on Arkham, ahead of the Mt. Gox Hacker.”

Last year, researchers revealed that another hacker group known as Citrine Sleet exploited a vulnerability in Chromium-based browsers and stole $3 billion worth of cryptocurrency.