Takahiro Shiraishi was arrested on October 31st on suspicion of corpse abandonment.
Investigators found the dismembered bodies inside seven cooler boxes and containers at his apartment in Zama City.
The suspect reportedly said he tried to dispose of the remains, but couldn't because he was afraid of being caught.
Police went to the apartment searching for a 23-year-old woman from Hachioji City. She had been missing for about a week.
Police say the two possibly met through a 'suicide website'. The suspect reportedly said he killed the woman when they first met.
Investigators quote Shiraishi as saying that he took money from several of the victims. The largest amount is reportedly 500,000 yen, or roughly 4,400 dollars.
Shiraishi also confessed that he had planned to sexually assault some of them.
Suicide-related online messages have led to crimes in the past
In 1998, a woman in her 20s killed herself with cyanide she received from a man she met online. He was later found to have given the chemical to other people. The man later took his own life.
In 2005, police arrested Hiroshi Maeue for allegedly killing 3 people he met online. He got to know his victims through a suicide-related website and suggested they commit suicide together. He has been executed.
2 years later, another man was arrested on suspicion of killing a 21-year-old woman he met online. She had reportedly told him she was afraid of dying alone and asked him to kill her.
People hoping to kill themselves together also often meet online.
In 2004, 7 people between the ages of 20 and 34 committed suicide by burning charcoal inside a rental car.
This past August, 3 people attempted suicide at a port in western Japan after soliciting each other on a social networking site. Police arrested 2 men for allegedly trying to assist, but later decided against indicting them.
In 2016, the Internet Hotline Center -- a private group commissioned by the police -- received 257 reports from the public on online postings encouraging people to commit suicide. Center officials say measures they take after being alerted include asking website operators to delete the postings.
Lawyer Hisamichi Okamura says as it gets easier to send information online, it is also getting easier for people to meet and get to know each other.
He says that in the past, people had to access each other's websites to post messages. But now all they need is a smartphone at their fingertips. Online tools, such as hashtags, help bring people with similar interests together.
Okamura says internet providers and social networking site operators have set rules allowing them to delete questionable messages. But he says it is impossible to remove them all, as the number of messages continues to skyrocket.
He says new online media keeps emerging, one after another, a perpetual cat-and-mouse game.
Social network operators: Measures are in place to prevent suicide
Social networking site operators say they have taken steps to prevent suicide.
Twitter bans users from posting information on how to commit suicide and from soliciting people to do it together.
But Twitter officials say that while tweets about suicide are against company rules, they should be handled differently from other violations. The officials do not freeze accounts or ask users to delete the posts. They say they need to make it possible for people around such users to detect signs and reach out to them. They say they refer the users to counseling services.
Facing allegations that the woman from Hachioji got to know the suspect through Twitter, the officials say they will continue talks in and outside the company to improve countermeasures.
Another major social network, Facebook, says friends of users who hint at suicide can file reports. When the users next access their accounts, a message appears saying "Someone is worried about you after reading your posting." The firm also refers them to counseling services.
But company officials say they do not remove messages soliciting people for joint suicides. They say the site requires people to register with their real names, discouraging users from using the platform to search for suicide partners.