Two of the boy’s relatives – a man and a woman – were arrested on murder charges, but the woman was released around 7pm Sunday evening.

“She was released after interrogations. The man remains in custody, suspected of murder, and a remand hearing will be held on Monday,” Ebbe Nyberg, duty officer at Västerbotten police force, told the TT news agency.

The police are keeping a tight lid on the circumstances of the event, which has been described as “a very tragic family drama”.

“Severe mental disorders are nearly always involved in this type of crime,” said criminologist Mikael Rying, of Mittuniversitetet, to TT.

A neighbour alerted the police, at around 8am on Sunday morning.

“Someone heard pleas for help, about calling the police, from an apartment,” said Dan Andersson, officer at the communications centre.

On arriving at the apartment, police officers found the severely wounded boy. He was taken to the hospital, but later died of his injuries.

According to criminologist Mikael Rying, an average of three children younger than 15 are subjected to deadly violence every year.

This figure has been more than halved since the 1990s, when 10-12 cases were reported per year. However, the figure can also vary greatly from year to year.

“When children are this badly hurt, one of the parents is the perpetrator in 80-90 percent of the cases. The crime is committed in their apartment, and the motive is rarely directed at the child. Instead, mental illness or custody cases can lie behind the violence,” said Rying.

The most common group of children to fall victim are the very young, around one-years-old.

“But the sort of case found in Umeå is thankfully very uncommon. Weapons are rarely involved in cases with young children,” said Rying.

The police have interviewed witnesses and neighbours in the area. The murder weapon has been found, and the apartment has been cordoned off.

A forensic investigation of the crime scene has also been carried out, and further investigations are ongoing on Monday.