The body parts of eight females and one male were found in cold storage cases in his apartment.
"It's true that I tried to hide the bodies of the people I killed to destroy evidence", Mr. Shiraishi said, according to a police statement cited by the Japan Times.
Japan has one of the lowest crime rates in the world. The brother later used her Twitter account password and found that she had been exchanging messages with Shiraishi, investigative sources said.
In July last year, a 27-year-old former employee at a care home for the disabled killed 19 and injured more than 20 in a knife attack in what is believed to be the deadliest mass killing in postwar Japan.
Photographers surround a police van transporting murder suspect Takahiro Shiraishi.
A bloodcurdling discovery was made in the apartment in Zama, a city in Kanagawa Prefecture, during an investigation into the disappearance of a young woman who went missing earlier in October.
They are now working to identify the victims, fearing that she might be among them.
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Shiraishi told the police that he later invited the woman to come alone to his apartment, where he killed her.
Her brother contacted the police on October 24, saying she had been missing since October 21.
The suspect told the police that he had sent a Twitter message to the woman saying, "Let's die together", Kyodo reported. She agreed to contact Shiraishi and to invite him to meet her at a train station while the police observed.
Shiraishi's neighbors expressed shock and disbelief at the news that nine bodies had been discovered in his apartment. When they asked Shiraishi if he knew where the missing woman was, according to the Asahi report, he replied, "In that cooler".
"I thought it smelled like sewage", he said.
Shiraishi's case is not the first involving the killing of suicidal people identified online.
What throws weight behind the confession is the discovery of a saw from Shiraishi's room.