In a late night press conference at the Tokyo Stock Exchange on Friday, Coincheck Inc.'s co-founder Yusuke Otsuka said that the company didn't know how its 500 million NEM tokens (a type of cryptocurrency) went missing, but the firm is working to ensure the safety of all client assets, reported Bloomberg.

It has suspended all withdrawals and halted trading of all tokens except Bitcoin for the time being. It has also stopped deposits into NEM coins.

The loss of NEM tokens can be valued between $400 million to $534 depending on its value at the time it was lost and the time the market reacted to the loss. The coins were lost after it was illicitly sent outside its venue. This kind of collapse in a cryptocurrency exchange comes after four years of Mt. Gox.

Mt. Gox, also a Japanese cryptocurrency exchange was the biggest Bitcoin exchange in the world handling 70% of all Bitcoin transactions before it had to file for bankruptcy in February 2014, after it reported that 850,000 bitcoins were stolen from its exchange.

Japan is known to have been taking measured approaches to regulate its crypocurrency markets to avoid another MT Gox incident. Questions are now being raised on the country's security practices of its exchanges.

Coincheck said that it had only detected the unauthorized activity at 3 am on Friday.