The alarm was raised by the 18-year-old son of the family who returned home on Friday afternoon to find his 57-year-old father, 55-year-old mother and 24- and 22-year-old sisters shot dead.
The man was in the hallway, the woman in the bathroom, and the two young women in a room an attic room.
Detectives say they are not ruling out either a crime of passion, nor the potential involvement of a stranger in the killings, and stress they are investigating in all directions.
It has been confirmed that the teenager is a member of the local shooting club. A weapon that might have been used in the killings has not yet been found.
The 30-strong investigation team is appealing for witnesses to come forward with information.