Police in Japan who have for months been taunted by an anonymous hacker have found a digital memory card attached to an animal's collar after solving a set of emailed riddles, reports said Monday.

The discovery was made after messages were sent to newspapers and broadcasters, with the sender claiming details of a computer virus were strapped to a cat living on an island near Tokyo.

The development is the latest in a bizarre investigation which has previously seen threats made against a number of venues -- including a school and a kindergarten attended by grandchildren of Emperor Akihito -- sent from computers around the country.

Japan's well-resourced National Police Agency (NPA) was embarrassed after it emerged officers had extracted "confessions" from four people who had nothing to do with the emails.

Police held one of the suspects for several weeks before a broadcaster and lawyer received another anonymous message containing information that investigators conceded could only have been known by the real culprit.

The four were released as the NPA chief issued a humiliating climbdown, acknowledging they had been the victims of a hacker, and promising his cyber-crime unit would brush up its skills.

A new barrage of messages was sent to media outlets on New Year's Day.

"This is an invitation to a new game," the email said, according to the Sankei Shimbun daily, one newspaper to receive it.

The email offered journalists the "chance for a big scoop", and contained a number of quizzes, the answers to which suggested a mountainside somewhere near Tokyo.

Investigators believe the same person or persons is also behind messages received on Saturday that led detectives to the cat carrying the mystery memory card.

It is not yet clear what the aim of the messages is, but commentators have speculated the culprit may enjoy toying with investigators and media.