Detectives are trying to find out how a 19-year-old man ended up with a "nasty" head injury and bleed on the brain.
Ryan Spencer knocked on the door of a house in Cotmanhay Road, Ilkeston, on 26 April, asking for help, and was taken to hospital for major surgery.
He spent two days unconscious and woke up at the Queen's Medical Centre with no memory of how he got the injuries.
Officers have been trawling through CCTV and carrying out inquiries to find out what happened to Mr Spencer.
Derbyshire Police believe he may have been on a bicycle when left his home some time after 02:00 BST on 26 April.
His rucksack was found by the Tesco store in Challon's Way but it appears that nothing was stolen from it, the force said.
Det Con Claire Croll, who is investigating the incident, said: "Ryan has no knowledge of how he received these injuries and has no recollection of the events from two days before until waking up in hospital.
"Clearly Ryan has received nasty head injuries and we are very keen to know how he has received these injuries."
Anyone with information has been urged to contact the force.