In the latest killing, an 18-year-old youth told police he randomly chose a person to kill and watched as Kariya, a civil servant, was hit by the 11.05pm train as hundreds of horrified commuters looked on.

To his incredulous interrogators, the youth said: 'I thought I can go to prison if I kill someone. It didn't matter who it was. I just pushed the back of the first person in front of me.'

The youth graduated from high school this month. He had a fruit knife on him when he was arrested. 'I was going to stab someone,' he told investigators.

'Obviously these are all individual cases and there will be different factors in each of these incidents, but it is deeply worrying to see this sort of psychopathic behaviour by young men,' said Makoto Watanabe, a lecturer in media and communications at Hokkaido University.

'The underlying problem appears to be the lack of face-to-face communication within families and communities, which leads some people to feel marginalised and not acknowledged as belonging.'

The family used to be the bedrock of Japanese society, with several generations often living under the same roof and youngsters absorbing their parents' and grandparents' values.

With more young people living on their own and the lives of countless others revolving around 'friends' they met only in cyberspace, a growing number were apparently unable to cope with modern society.

'They have a sense of loss, of being cut out of society, and in a distorted way believe that by committing a serious crime they will get society to pay them attention,' Professor Watanabe said.

Tuesday night's attack came a day after a young man riding a bicycle in Nagoya stabbed a woman leaving the restaurant where she worked, and, even more shockingly, a rampage at a train station on Sunday that left one person dead and seven wounded.

Arrested in the town of Tsuchiura, north of Tokyo, Masahiro Kanagawa, 24, told police: 'It didn't matter who they were. I planned to kill seven or eight people.'

Four days earlier, Kanagawa had allegedly knifed to death Yoshikazu Miura, 72, and was being hunted by police.

Kanagawa did not know his victims and it is the indiscriminate nature of the attacks that has caused consternation and confusion in a society that still prides itself on being one of the safest in the world.

Police believe Kanagawa acted out of a sense of isolation and reportedly rarely left his home, where he constantly played computer games after leaving his part-time job in a local convenience store in January. He told authorities he had planned to kill his younger sister because she often argued with their mother, but had been unable to find her.

He added that the family never ate together, he did not know their mobile phone numbers and none of them had talked to him recently.

'There has been a clear increase in the number of cases like this in the last few years, but also in the severity of the attacks and the madness of the perpetrators,' Professor Watanabe said. 'I fear it indicates the depth of the social crisis we face in Japan, the breakdown in human communications and the destruction of the family and local community.'