Authorities in Poland are investigating the closure of Bitcurex, a long-running bitcoin exchange in the country.

Launched in 2012, Bitcurex has processed over $50 million worth of bitcoin in Poland in 2016. But last October, the bitcoin exchange suddenly went offline. In a statement, Bitcurex informed its users that it has “encountered problems with an update,” prompting the company to “temporarily suspend the exchange as they work towards restoring service.”

At the time, the company did not mention a hack or loss of funds. But a week later, the exchange confirmed that its IT system was “damaged by an external interference,” resulting in the loss over 2,300 bitcoins worth more than $1.5 million at the time.

Bitcurex said it had already notified the authorities of the incident, which could be a potential crime.