The bodies of two adults and two children were discovered, hacked into pieces in a house outside Madrid, police and the local mayor told AFP.
The child victims were aged "around 4 and 1", a police spokeswoman said, adding that the gruesome discovery was made at a house just outside the village of Pioz, around 60km northeast of the Spanish capital.
All four victims were members of the same family, local Mayor Ricardo Garcia Lopez told AFP.
The mayor added that, according to neighbours, the family were renting the house and were not Spanish nationals, but Latin Americans.
Police said the family living at the house were of "Brazilian origin" but added that the autopsy results would be required to confirm whether the bodies found were those of the occupants.
The alert was sounded in the morning by one neighbour who smelt a strange odour emanating from the house, police said.