Japanese police are today investigating whether the man who stabbed seven people to death yesterday in Tokyo had given an online warning of his actions.
Police have arrested a man (25) who they said drove a truck into a crowd of people, then got out and began a frenzied knife attack.
It has been reported that he had warned on a mobile phone messaging board on Sunday that he was heading to Akihabara, Tokyo's biggest electronics shopping district, to kill people.
The suspect in Sunday's attack lived alone and had a temporary job at a car factory, media said.
Passers-by today prayed and placed flowers on the Tokyo shopping street where seven people were killed yesterday in a stabbing rampage that Japan is struggling to make sense of.
The killings follow a string of similar incidents in recent months, shocking Japan and sparking talk of failing communities and declining morality in a country proud of its low crime rate.
One person was killed in a random stabbing outside a train station north of Tokyo in March, and five were hurt in a similar attack in January. Also in March, a teenager pushed a stranger under a train in western Japan, saying he simply wanted to kill someone.
"Japan has entered a period of selfishness. People have the feeling that they can do anything," said Jinsuke Kageyama, a criminal psychologist at Tokyo Institute of Technology.
"But when these people fail to fulfil themselves in socially acceptable ways, they are treated as losers and their frustration builds up," he added. "A series of disappointments can lead them to try to regain their sense of self through crime."
Total reported crime has been falling for five years, but Japan has toughened up sentencing and increased the pace at which it carries out executions in recent months under pro-death penalty Justice Minister Kunio Hatoyama.
Japan's top government spokesman said it was hard to pin down the reason for Sunday's attack but that tighter controls should be considered on survival knives like the one the attacker used.