The children were found seriously injured at an apartment block in the Hässelby suburb shortly before 10pm on Sunday.
An investigation was launched and two people, a man and a woman, were in custody on Monday morning on suspicion of murder and attempted murder.
“We cannot rule out that the children have been subjected to a criminal offence,” Stockholm police spokesperson Helena Boström Thomas told the TT news agency.
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Police would not confirm the details, saying only it was unclear what was behind the incident and that the children were believed to have “fallen from a great height”.
The Expressen tabloid cited unconfirmed reports that the father allegedly stabbed the children and then threw them out the window, before stabbing himself. An injured man in his mid-40s was found in the apartment. Note that this was an ongoing investigation on Monday morning, still in its early stages, so unconfirmed information may change as further details emerge as part of the probe.
“Two adults with a close relationship to the children were arrested during the night, but I cannot confirm what the relationship between the adults and the children was,” a police control room officer told The Local on Monday.
He would not confirm reports that the children were pushed from the window, but said they were investigating the incident as suspected murder.
The children, younger than ten, were taken to hospital with serious injuries. At 8.30am on Monday, police confirmed in an update on their website that one of the children had died and the other was still in hospital with life-threatening injuries.