Coincheck confirmed late Friday that the unknown hackers might have stolen some 500 million NEM tokens (worth up to ¥58 billion or around $532 million at the time of the incident), which makes it the biggest ever theft from a cryptocurrency exchange. The previous record-holder was Mt.Gox, a world leading bitcoin exchange, in February 2014 when some 850,000 bitcoins (worth $390 million at the time) were stolen from the platform, preceding its eventual demise.

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This time the heist affected only NEM [coin symbol XEM] cryptocurrency, which is currently the 10th largest virtual currency according to CoinMarketCap. XEM was created by NEM foundation and is based on a blockchain technology. Since the incident become public, the token price plunged more than 15 percent from the day’s high of around $1.01 down to $0.84.

The illegitimate withdrawal of the cryptocurrency was discovered on Friday morning. Coincheck rushed to suspend all withdrawals of NEM, as well as sales and purchases. By Friday evening, the ban on withdrawals was extended to all currencies. Trading has also been halted for all cryptocurrencies, expect bitcoin.

Coincheck CEO Koichiro Wada and Chief Operating Officer Yusuke Otsuka said it is so far unclear who was responsible for the hack, how it was perpetrated and what damage has been done.

Police and the Japanese government’s financial regulator, the Financial Services Agency, have been notified of the breach, they said. Police cars were spotted at the exchange’s offices late on Friday. 

Coincheck has offered its “sincere apologies” to the affected customers but fell short of blaming the incident on lax security measures. “We are deeply sorry for troubling people with this issue,” Wada said, as cited by Japan Times.