One of Japan's largest digital currency exchanges says on Saturday it has lost some $534 million worth of virtual money in a hacking attack on its network.
Coincheck suspended deposits and withdrawals for all crypto-currencies except Bitcoin as it assessed its losses in NEM, a lesser-known coin.
If the theft is confirmed, it will be the largest involving digital currency.
Another Tokyo exchange, MtGox, collapsed in 2014 after admitting that $400 million had been stolen from its network.
The stolen Coincheck money was said to be kept in a "hot wallet" - a part of the exchange connected to the internet. That contrasts with a cold wallet, where funds are stored securely offline.
Coincheck says it has the digital address of where the money was sent and is going to do what it can to compensate investors.
Hackers broke in at 02:57 (17:57 GMT) on Friday, the company said in a statement.
However, the breach was not discovered until 11:25, nearly eight and a half hours later, according to BBC.
Company chief operating officer Yusuke Otsuka said 523m NEMs had been sent from Coincheck's NEM address during the breach.
"It's worth 58 billion yen based on the calculation at the rate when detected," he said at a press conference at the Tokyo Stock Exchange.
Coincheck was still examining how many customers had been affected and trying to establish whether the break-in had been launched from Japan or another country.
"We know where the funds were sent," Mr Otsuka added.
"We are tracing them and if we're able to continue tracking, it may be possible to recover them. But it is something we are investigating at the moment."
Coincheck reported the incident to the police and to Japan's Financial Services Agency.