Japan has seen several serious youth crimes in recent years

The student was identified from security camera footage of a young man walking with the victim, Shun Tanemoto, on the evening of the killing, according to Japanese media reports.

Shun Tanemoto went missing from an electronics shop in the city on 1 July, after leaving his family to go to the store's video game section.

The following day, his naked body was discovered next to a multi-storey car park, just 4 kilometres (3 miles) from the shop.

Police believe he was thrown to his death from the top of the car park.

The youth seen on camera with the victim was wearing a white, short-sleeved shirt, black trousers and white shoes - an outfit resembling the uniform of one of the city's junior high schools, according to media reports.

Police believe the suspect may have also been involved in other recent cases of abuse concerning young children.

Two boys were molested and another assaulted in April at a commercial complex not far from the electronics store, according to the police.

Commenting on the Nagasaki case, Chief Cabinet Secretary Yasuo Fukuda said the issue was of concern to everyone in Japan.

"We have to think deeply over whether this is only an issue of the boy or of society as a whole," he said.

Several incidents of violent youth crime have shocked Japan in recent years.

In 1997, an 11-year-old boy in the city of Kobe was murdered by a 14-year-old boy, who cut his victim's head off and left it outside a school gate.

Last weekend, three teenagers were arrested in Chatan for allegedly beating a 13-year-old schoolboy to death and burying his body in a cemetery.

Under Japan's Criminal Code, children under 14 cannot be criminally prosecuted.

Suspects are instead transferred to child welfare centres, which then decide whether to refer the case to a family court.