Computers at Mitsubishi Heavy Industries (MHI), Japan’s largest military defense contractor, were infected with at least 50 different kinds of virus, the company found during an investigation into a cyberattack it suffered earlier this year.

Japanese newspaper Yomiuri Shimbun reported today that MHI found the extra viruses while checking the 83 computers that had been compromised in the attack. The number is significantly greater than the eight viruses that were first reported. 

When it first reported the attacks on MHI, Yomiuri reported that the viruses may have been created using Chinese-language scripts. MHI, which builds submarines, missiles and components for nuclear power plants, said at the time that company information many have been stolen, but noted that "crucial data about our products or technologies have been kept safe".

Following the attack, Japan’s Ministry of Defence asked 20 defense contractors whether they had been attacked in the past. Eight, including Mitsubishi Electric, Kawasaki Heavy Industries Group and IHI Group, admitted that they had seven said they had not, and five others did not reply.

Other Japanese defense companies have also admitted to being targeted by cybercriminals. Mitsubishi Electric, Kawasaki Heavy Industries Group and IHI Group have all come forward, according to Yomiuri, but other companies have been more cautious.

Japan’s government held an emergency meeting of its Information Security Policy Council on Friday, with the aim of applying increased cybersecurity efforts across government.