After receiving a neighbour’s call on Sunday night, cantonal police said they responded to the home where they found the girl, her severely wounded 44-year-old father and her slightly injured 43-year-old mother.
The girl was taken along with her parents to hospital but she was dead on arrival, police said on Monday.
Investigators maintain the exact circumstances of the killing remain unclear but they don’t believe a third party was involved.
A neighbour told newspaper 20 Minuten that he heard the girl’s mother run out of the house and cry “Help, my husband has killed our daughter” several times.
The 23-year-old man said he saw ambulance attendants carrying the father in a stretcher with two large kitchen knives “stuck in his belly”, the newspaper reported.
He said the mother had a blade in her head and collapsed.
The neighbour and his sister said they found it difficult to believe what had happened.
“He (the father) seemed to be a loving father and brought his daughter regularly to kindergarten.”
However, neighbours noted the family was withdrawn and did not participate in neighbourhood get-togethers, 20 Minuten reported.
Police noted that they were unaware of any prior problems with the family.
The Lenzburg-Aarau prosecutor’s office has launched an investigation in cooperation with cantonal police.