Bitstamp co-founder Damijan Merlak told the STA Tuesday he could not comment on the reports.

The investment was reportedly carried out late last year, before Bitstamp, which is officially based in London, became the biggest bitcoin exchange in the world in February following the closure of Japanese Mt.Gox.

According to Bloomberg, this is one of the largest single investments in a bitcoin company and was a part of Fortress's plan to build a brand in the world of bitcoin.

Turnover at Bitstamp reached 15,000 bitcoins or nearly $9m in the last 24 hours. According to some estimates, the company generates around half a million dollars in revenue each month.

Bitstamp was founded by Merlak and Nejc Kodrič in 2011, when bitcoin was worth less than $5. Today the price for one bitcoin is at around $600.