Takahiro Shiraishi, the Japanese man who has reportedly confessed to murdering and hacking up nine young people in his Tokyo bathroom, was said to be a quiet schoolboy who would grow up to work on the fringes of the sex industry and become a suspected serial killer.

Pictures of the 27-year-old show an ordinary-looking man with neat, dark hair and glasses, who lived in a nondescript flat on a quiet residential street in one of Tokyo's endless sprawling suburbs.

But on the morning of Halloween, police uncovered a grisly house of horrors behind Shiraishi's front door: nine dismembered bodies with as many as 240 bone parts stashed in coolers and tool boxes, sprinkled with cat litter in a bid to hide the evidence.

People who lived in the neighborhood remembered the young Shiraishi as a "quiet child who was able to socialise with neighbours".

At school, his grades were far from stellar but he was an attentive pupil, who "didn't especially stand out but was not a gloomy character either", according to a former classmate cited in the Asahi Shimbun.

He enjoyed athletics and baseball and was "a good listener rather than someone who would speak about himself", another school contemporary told a local Tokyo paper.

One person claiming to be a former schoolmate took to Twitter saying he was so "normal, inconspicuous and low-profile" that most classmates would not even recognise him when news of his alleged crimes broke.

But the warning signs were perhaps there as one elementary-school contemporary told the private Fuji TV network that Shiraishi and his friends enjoyed choking each other for "fun".

"He once passed out while playing the choking game," the man, who did not wish to be identified, told the show.

The Mainichi Shimbun has reported that two of the bodies showed signs of strangulation, one had broken neck bones and another had bleeding patterns typically associated with choking.

'Watch out!'

After graduating from high school in 2009, Shiraishi got a full-time job at a supermarket but quit just over two years later.

At that point, he began to work as a scout for sex parlours in Kabukicho, Tokyo's biggest red-light district, seeking to lure young women into working in the clubs there.

In February, he was arrested and eventually handed a suspended jail sentence for recruiting a young woman for a sex shop in the full knowledge that she would be pressed into prostitution.

Several people tweeted about a "creepy scout", with one person apparently employed in the same business as the suspect posting a photo of him with the caption: "Watch out for this scout."

Shiraishi appeared to have a close relationship with his father, a designer of automobile parts, after his mother and younger sister left to live closer to the girl's school in central Tokyo.

And a woman who said she was in a relationship with him until summer 2016 described him as a "gentle character" who was "never angry with women."

"When I told him that I wanted to break up, he hugged me and said something like 'Don't go'," the women told Fuji TV.

'Professional hangman'

Things started going downhill for Shiraishi around June this year when he is reported to have told his father: "I don't know why I'm alive."

On August 22, he moved into the one-room apartment in Zama, a southwestern suburb of Tokyo, that would become the apparent scene of multiple murders.

He set up several Twitter accounts, advertising himself as a "professional hangman" and contacting young women who said they had suicidal tendencies.

He is reported to have told investigators that he killed his victims "as soon as he met them" and then "did some work on the bodies" to cover up his alleged crimes.

According to the Japan Times, he told police that the first time he dismembered a body, it took him three days but that "from the second person, I was able to do it within a day."

Several media report the police found scissors, knives, a saw and woodwork tools in his flat.

One former hostess said on Twitter she had a lucky escape after her parlour turned down Shiraishi, who wanted to take her on a "date".

"The parlour politely declined it but if it hadn't, I would have been dead, wouldn't I? I get chills," tweeted the woman.