Calligraphy teacher Ritsuko Tada was used to the quiet life in small-town Hokkaido, but all that changed when one of her students was arrested on suspicion of murdering a female colleague.

"I was shocked by her arrest, but I could not believe she had been involved in the cruel murder," said Tada, 52, who also runs a coffee shop in Hayakita, a town of 5,600.

Her belief in the innocence of her longtime student, Minako Okoshi, has deepened by her feeling that the investigation carried out by police and prosecutors was "unreasonable," she said.