Up to 15 boys aged 12 to 14 have been questioned as police investigated a Jamie Bulger-style murder that has shocked Sweden.

Kevin, aged four - whose last name has not been released by the police - was abused, bludgeoned to death and thrown into a lake last month. Police believe the culprits are three boys between 12 and 15.

The child was playing with a friend near their homes in Arvika, south-west Sweden, when the older boys attacked him. Kevin’s injuries were said to be the result of “sexually-conditioned violence”.

Kevin’s friend is deeply shocked and unable to provide a clear picture of what happened.

One 12-year-old has already confessed to the murder, though police are awaiting the results of forensic tests.

Most of the 15 boys who are being questioned were fetched from their schools last week and interviewed individually in the presence of a social welfare officer, but without their parents.

The Arvika case, the first fatal instance of violence among children since 1982, was preceded in July by the murder in of a 58-year-old Gothenburg man by four children, the youngest aged just 12.

A survey carried out for the Swedish Council for the Prevention of Crime shows that child abuse by children is increasing. There are currently 200 to 300 cases a year, with a further ten to 12 of violence with a sexual element between children under the age of 15. The age of criminal responsibility in Sweden is 15; offenders below that age are normally handed over to the social welfare service.

“This compels us to think about how we can stop such fearful things happening in our society,” said Laila Freivald, the minister of justice. “How do we prevent children becoming so violent?” Some specialists say that children who are lethally violent have almost always been subjected to strong trauma themselves - from the violence of war to violence in the home. If the child also lives in a so-called toxic environment, with no adult to offer warmth, support, guidance and hope, the risk of violence is greater. Particularly at risk are children rejected by both parents and children of their own age.

“Rejected children often seek other rejected children and form groups with hierarchies,” says Sven Hessle, a Stockholm professor of social work. “Their self-restraint often fails and they do things they would never do alone. ”