Two children were found with serious injuries in front of an apartment building in Hasselby, north-west of Stockholm, late on Sunday.
The police suspect that they fell from a great height. One of the children died of the injuries in hospital on Monday. The police suspect that they were victims of a crime.
A man and a woman are now being investigated for murder and attempted murder, the prosecutor's office announced on Monday.
Swedish news agency TT reported that all four are related. Both children are said to be younger than 10 years old.
