Japanese authorities began investigating Mt Gox, the Tokyo-based Bitcoin exchange that shut down this week, amid calls for regulation of the digital currency as customers face losses.

The Financial Services Agency, Finance Ministry and police are examining the closure of Mt Gox, chief cabinet secretary Yoshihide Suga said today.

Mt Gox has been subpoenaed by US prosecutors, the Wall Street Journal reported.

Mt Gox’s website went offline yesterday as a document that appeared to be an internal strategy paper surfaced on the Internet saying it lost 744,408 Bitcoins, valued at more than $390 million.

The news triggered renewed calls for more security and consumer protection from US officials, while Democratic Party of Japan lawmaker Tsutomu Okubo called on his government to rectify the lack of regulation for the virtual currency.

“The authorities’ stance that they don’t have responsibility is disgraceful,” Okubo, a former vice finance minister who also used to work at Morgan Stanley, said in a telephone interview. “We need to establish a legal system.”

Bloomberg