Although the main suspect is now in custody, investigators say the latest developments are raising more questions than providing answers.
The incident took place in the small town of Mistelbach — located near the southeastern city of Bayreuth.
The bodies of a 47-year-old woman and her 51-year-old husband were found in the basement. Police said the two sustained stabbing wounds.
The couple lived in the house along with their four small children — who are all under the age of 18.
Authorities said the children were in the house at the time of the killings — but that it is not yet clear how much of the crime they witnessed.
Neighbours called authorities shortly after midnight on Sunday after hearing cries for help from the house.
Police launched a large manhunt for the suspect — who later turned himself into a police station around 04.00.
Authorities said they are working to determine a motive for the killings — but the reason for the alleged attack as well as the teen’s connection to the family remains unclear.
The 18-year-old male suspect is not a relative, a police spokesperson told German news agency DPA. Initial investigations indicate that the teen was a guest and that he had spent the night at the couple’s house.
The 18-year-old is due to appear before an investigating judge on Monday and remains in custody.
The couple’s children have also been questioned by police and are being cared for by local youth authorities