The mother of a 16-year-old girl who died from emaciation following lengthy detention and menacing interrogations by Japanese authorities is seeking compensation, in the latest “hostage justice” case.

The lawsuit, filed with western Japan’s Kobe district court on Wednesday, argues that the girl’s 18-day ordeal at a jail traumatised her so much that five months after her release, she weighed just 20kg (44lbs) – and died in December.

“My daughter was unrecognisable when she died,” the mother of Runa – a pseudonym used in the complaint – told a news conference on Wednesday.

“I want to know what happened to her, and why she was arrested, detained and had to die,” the plaintiff said.

In Japan’s harsh criminal justice system, critics say innocence is not always presumed and that authorities rely heavily on confessions.

Campaigners say suspects’ decisions to remain silent or deny charges often lead to lengthy, gruelling pre-trial detention, with confessions used as a de facto condition for release – hence the term “hostage justice”.