Tomohiro Kato, 25, clad in a T-shirt and with his head down, was driven by van out of a police station in the Tokyo district of Akihabara as dozens of camera crews swarmed around.
In Japan's deadliest crime for seven years, Kato drove a rented two-tonne truck Sunday into Akihabara, a crowded district regarded as an electronics and sub-culture hub.
There, he swerved the vehicle into pedestrians, jumped out and slashed at random with a survival knife, wounding a total of 17 people - seven of whom died - before being overpowered by police.
Kato told police he was "tired of living" and simply wanted to kill.
Prosecutors are now expected to seek judicial permission to detain him for 10 days for interrogation, which can be extended for another 10 days, as they decide on charges.
If charged with murder and found guilty he could face the death penalty.
Japan's education minister Kisaburo Tokai said Tuesday the killings made him "angry."
"It made me think about whether the background of this incident may have been linked to education," he told reporters.
Tokai said he was considering holding talks with experts on brain science about the state of children in Japan.
Kato was a model student and had a good reputation with adults during his elementary and middle school days, local residents in Aomori, the northern city where he grew up, told Japanese media.
He went to an elite high school but became increasingly quiet and mediocre in his studies, they said, eventually taking manual work at an auto components factory.