The prosecutor's office in nearby Hildesheim revealed that the man had suffered serious injuries "presumably" from trying to commit suicide, and was currently in an induced coma in hospital.
Marital problems are thought to have been the motive. The 36-year-old man informed his wife, abroad on holiday at the time, of the crime via text message. She then passed the message on to relatives, who contacted police.
Officers subsequently stormed the family's apartment on a housing estate on Thursday night, where they discovered the children's bodies and the seriously injured father, along with a suicide note that he had left.
The prosecutor's office told The Local that the children had suffered "cut injuries," but it is thought they were asleep at the time of the attack.
The father will be arrested on suspicion of murder when he awakes. Investigators are still searching for the murder weapon.
Despite the presumed motive, there is no indication that the couple were separated. The prosecutor's office says the man was not previously known to police, and described the family as "completely ordinary."
According to reports, the mother has now returned to Germany. Autopsies are expected to be performed on Friday.