Officers were called to a flat in Coventry Road, Market Harborough, at around 3.45pm on Saturday April 21.
A 19-year-old woman sustained a single stab wound and was taken to hospital by air ambulance.
The woman, who suffered from serious injuries, remains in hospital in a stable condition.
The flat and a number of other flats in the immediate area were cordoned off while officers carried out enquiries at the scene. The cordon has now been lifted.
A 33 year old man and a 37 year old woman, both from Market Harborough, were arrested in connection with the incident. They have since both been released under investigation.
Detective Sergeant George Fraser said: “We are currently carrying out enquiries to establish the full circumstances of the incident.
“Anyone who was in the area at the time and who may have seen anything unusual or have any further information regarding the incident is asked to please get in touch. Your information could help our investigation.”
Anyone with any information is asked to contact 101 quoting 18000180274.