Secret documents on the Air Self-Defense Force's aircraft control and warning radar system have recently disappeared at the Misawa ASDF base in Aomori Prefecture, Defense Agency officials said Thursday.

ASDF officials checking the security of key information found on Jan. 18 that classified documents kept in a safe at the Northern Air Defense Force base were missing, the officials said, and a later search failed to find them.

The agency officials said they believe the document could have been mistakenly shredded, adding that they do not think the information was leaked to outsiders.

According to the agency, the missing documents were assigned midlevel secrecy among the three categories of confidential internal documents. The officials said the document concerns "aircraft control and warning activities by radar" but did not disclose further details.

The agency imposes various restrictions in handling such classified documents, including keeping them in safes and prohibiting officials from taking them out of the offices. Documents with designated periods of storage also carry instructions on whether they are to be sent to authorities within the Self-Defense Forces or destroyed in the presence of officers in charge, according to the agency's rules.

The missing documents were scheduled to be brought back to the Air Defense Command based in Saitama Prefecture, the officials added.

The daily Sankei Shimbun reported in its Thursday morning edition that the documents may have included data on the operation of a device to identify friendly and enemy planes. But the officials denied this, claiming the lost documents have nothing to do with the device.

The ASDF will consider punishing those held responsible for the poor management of classified information when the probe is completed.