A seriously injured woman found barefoot and in a nightdress in a Rochdale town centre alleyway had been attacked, police believe.
The 20-year-old victim was found by two women in a side street between Yorkshire Street and the town's police station in the early hours of Saturday.
She was taken to hospital for treatment to serious head and facial injuries and is in a serious but stable condition.
Detectives are now trying to establish where and when she was attacked.
It is not believed the woman had been at her home address and she had probably been staying elsewhere.
Det Insp Vicky Shaw, of Greater Manchester Police, said: "Unusually this woman was in a nightdress and barefoot when she was found and I am certain someone will have seen her at some point.
"Did you pass her on the street or spend time with her on Friday night or Saturday morning?
"It is vital we know what her last movements were so we can establish exactly who attacked her and when."