Mother said, 'I had stress due to parenting'
TokyoReporter
Top of the victim's skull was partially crushed
Suspect said, 'Japanese people are bad'
Suspect allegedly attempted to blackmail 37-year-old victim out of 4 million yen
Three of the suspects got to know the ringleader via a members-only site for 'underground part-time work'
Suspect met the woman on a so-called "konkatsu" app that aims to connect persons seeking marriage
Police said, 'It is regrettable that a staff member was arrested'
Suspect, a former employee, told police, 'I held a grudge from when my time working'
According to police, the suspect met the girl via a social-networking service
Two stones weighing about 1 kilogram each were also found inside a backpack carried by the woman