Reports indicate that more than 100 documents were leaked; the documents were dated between 2004 and 2010 and contained lists of people who cooperated with the Japanese police and US law enforcement in anti-terror activities around the world, as well as addresses and family information of the informants.
"Information was provided to the police that data related to international terrorism was placed on the Internet. Based on that information, Japanese police are conducting an investigation", a spokesman for Prime Minister Naoto Kan told the Wall Street Journal.
The disclosure appears to be an intentional act by someone within the Tokyo Metropolitan Police, according to the Asahi Shimbun newspaper. The newspaper said that leaked information contained information not only about informants, but also terrorism suspects.
The documents were leaked from a computer in Luxembourg and spread though the Winny file-sharing software, according to an IT source consulted by the newspaper. Luxembourg could have been used as a relaying point to disguise the origin of the leak, the source said.
In an English-language editorial, the Yomiuri newspaper wrote:
"The leak of documents thought to have originated with the police is an extremely dire event, and one that may shatter international trust in Japan's police force….If these materials turn out to be internal police documents, the impact would be immeasurable. It cannot be denied the informants could come under threat of physical harm, and it is highly likely they will no longer be willing to cooperate with police. In addition, foreign investigative organizations could become unwilling to provide Japan with information on international terrorist organizations out of concern such information may be compromised."