A series of public appeals were made and national law enforcement agencies were drawn into the manhunt which used a range of covert methods and surveillance techniques.

The covert methods used to get the evidence needed to run the story highlight problems and dilemmas papers face when conducting journalism in the public interest.

They needed more covert methods and were prepared to leave their fields and walk 100 miles for them.

The study says they employ a variety of covert methods, including travelling with false documents, concealing themselves in vehicles or by overstaying visas.

Their covert methods of waging war contrasted the ninja with the samurai, who observed strict rules about honor and combat.