Mystery surrounds how a man found unconscious on a busy walking trial sustained critical injuries.
Emergency services were called to Moonee Ponds Creek Trail in Glenroy, about 14km north of Melbourne's CBD, on Sunday afternoon after a passerby found the man moaning and unconscious.
He was treated by ambulance paramedics at the scene before being taken to hospital with critical injuries.
Police said the man, believed to be aged in his early 20s, did not have any belongings or identity documents with him and nothing of note was found at the scene.
After initially appealing to the public for help in identifying him, police were able to identify the man late on Monday morning.
"At this stage it is not believed he was assaulted, however the circumstances surrounding what has occurred is unclear. The man remains in hospital in a serious condition," a police spokesperson said.
One possibility police are exploring is he may have fallen or slipped from the Moonee Ponds Creek Viaduct, which sits above Kingsford Smith Ulm Reserve, where the man was found.
