TOKYO (BLOOMBERG) - Japan is investigating a report that a high-level cyber attack in September - possibly involving a state actor - may have stolen information from an internal military computer network.

Officials at Japan's Ministry of Defence and Ground Self-Defence Force said they were investigating a Kyodo news report on the attack. They asked not to be named due to ministry and military policy.

The hackers didn't leave a detailed trail and the extent of the damage is unclear, Kyodo said, citing ministry sources. The news agency said the hackers took advantage of the fact that computers at Japan's National Defence Academy and National Defence Medical College are connected both to a university network and to an internal network linking military bases.

The report also cited senior military officials as saying the attack was viewed as a crisis, and staff at the ministry and the Self-Defence Forces were temporarily banned from connecting to the Internet after the incident became apparent in September.

The reported attack came two-and-a-half years after the SDF set up their own cyber defense unit.

In 2011, a cyber attack on military contractor Mitsubishi Heavy Industries Ltd. was believed to have targeted defense technology, according to the Nikkei newspaper. Japan's space agency, JAXA, also suffered cyber attacks in 2013, and a naval officer was convicted in 2008 over the unauthorised sharing of information related to the Aegis missile defense system.