Satoshi Uematsu turned himself in following the stabbing spree in Sagamihara, Kanagawa Prefecture - Japan's worst mass killing in decades.
The 26-year-old today appeared before public prosecutors as a team of police officers raided his home.
About half a dozen plainclothes officers entered the property as reporters and TV cameras stood by.
Uematsu was sent from a regional jail in the town of Sagamihara, around 25 miles southwest of Tokyo, to the Yokohama District Public Prosecutors Office in Kanagawa prefecture.
Video footage showed him smirking in the back seat of a police car as it drove away.
Uematsu had said in letters he wrote in February that he could "obliterate 470 disabled people" and gave detailed plans of how he would do so, Kyodo news agency reported.
Uematsu was involuntarily committed to hospital after he expressed a "willingness to kill severely disabled people", an official in Sagamihara said.
He was freed on March 2 after a doctor deemed he had improved and was no longer a threat to himself or others, the official said.
The affair has shocked a nation where the crime rate is low and such mass killings rare.
It has also sparked debate on whether and how the system for involuntary commitment and aftercare broke down, since Uematsu had previously made clear his intent to commit the crime.
"Involuntary commitment is done forcefully by the authorities ... If the time period drags on longer than necessary, it becomes a serious violation of human rights," Asahi newspaper said in an editorial on Tuesday.
"However there were warning signs before the incident," the paper added. "Was the treatment and outwatch of the man sufficient? It is vital to closely examine the system of support for the man and his family, and the contacts between the medical system and the police."