The body of Maddie Durdant-Hollamby, from Wimblington near March, was found by police at a property in Slate Drive, in Kettering, shortly after 1pm on August 27, following a report of concern for her welfare.
The body of Benjamin Green, 41, was also found at the same property, which he owned, and it was previously confirmed that the two had been in a relationship.
Northamptonshire Police said preliminary post-mortem reports showed Ms Durdant-Hollamby died of stab wounds, and Mr Green of self-inflicted knife wounds.
The force said Ms Durdant-Hollamby’s death was being treated as murder and no-one else was being sought in connection with the incident.
It added that both families have been informed of the findings and continue to be supported by specialist officers.
Tributes were paid to Ms Durdant-Hollamby by her family on Sunday. In a statement they said: “We are absolutely devastated at the loss of Maddie, our beautiful daughter, sister, granddaughter and niece.
“Our hearts are broken and we cannot begin to imagine life without her. The shock and pain we are feeling is indescribable.
“She was a beautiful person both inside and out, who loved nothing more than spending time with her family and friends.
“Maddie was a kind, caring, thoughtful young woman, who touched the lives of everyone she met.
“A talented dancer, her creative talent continued through school and into her career in marketing. Her life has been sadly cut short just as her career was blossoming.
“Our thoughts are with everybody who knew and loved Maddie as much as we did.”
Senior investigating officer, Detective Chief Inspector Joe Banfield, said: “Our investigation into the deaths of Maddie and Ben continues, and a file is being prepared for the coroner.
“We continue to ask that the media leave the families and friends of Maddie and Ben to grieve in peace, and cease any efforts to contact them at this desperately sad time.”