A major newspaper in Japan, Yomiuri Shimbun cited an unknown source connected to the investigation stating that 99% of the missing bitcoin was based on internal system manipulation and this meant it was likely to be an inside job. The company Mt Gox have had their suspicions for a while that the theft of 650,000 BTC might be an inside job, however no one has named a suspect officially or unofficially.

The company hadn’t faced any major hack attacks, and the only person working full time was CEO Mark Karpeles, who was employing a series of contractors on temporary work assignments. During this time no one has claimed Karpeles to be responsible.

Mt Gox had originally released an official line saying the bitcoins disappearance was a gradual theft derived from the ‘transaction malleability’ flaw in bitcoin’s underlying code. This claim was reinforced by the core developer of bitcoin, Gavin Andresen, and other researchers.

Back in early February the company began using the transaction malleability line, before it was obvious the exchange had been damaged beyond repair.

In March 2014 a leaked transaction data showed anomalous behavior attributed to two automated trading bots who had been nicknamed ‘Willy’ and ‘Markus’. This was the biggest sign of impropriety within Mt Gox. Coincidentally the Yomiuri reported links to “suspicious accounts” but did not specify if they were these known accounts, or new ones.
‘Willy’ had appeared at certain intervals in late 2013 with what seemed like several different user IDs. Each ID seemed to show irregularities in their records, most commonly a “??” in place of user location. The bot was responsible for buying $2.5M bitcoins at the current market rate for that time then cease trading.

Similarly the bot ‘Markus’ came earlier and ‘bought’ bitcoins at random prices despite never seeming to have spent any actual fiat money on transactions. Collectively the bots bought roughly 570,000 BTC up until November 2013, at which point records are no longer available to the public.

One theory is the two bots continued their activity up until the implosion of the company, resulting in the disappearance of 650,000 BTC.