According to them, the interface of the software that the attackers used to remotely control the infected computers contains simplified Chinese characters for some of the functions, and this fact would make the program difficult to use to an attacker that doesn't speak or read Chinese.
There's always the possibility that these characters were used in order to deflect blame and place it one the shoulders of China, the usual suspect in cases such as these. Nevertheless, the Tokyo Metropolitan Police Department is consider this particular attack as an example of international espionage.
According to the news site, the records from the infected servers also showed that they were connected with Web sites in China and Hong Kong but, once again, that doesn't mean much.
Security experts were quick to speculate on how the attackers managed to get a foothold in the company's networks, and most of them seem inclined to believe that spear phishing emails containing malware sent to company employees were the most likely leading point of entry.
The theory is seemingly backed by the revelation that IHI - another defense contractor based in Japan - has confirmed that the number emails with malicious attachments targeting its employees has also surged.
And while the police is investigating the matter, the Japanese Government has expressed its disapproval about finding out about the breach from the papers and about the fact that even though it happened back in August and to the biggest defense contractor, the company has failed to inform the Defense Ministry about it.
Reading our newsletter every Monday will keep you up-to-date with security news.
Receive a daily digest of the latest security news.