Detectives are investigating how a bleeding man who took a bus to hospital came about his injuries.
Police said the 27-year-old boarded the First Bus 201 service on Westwood Road, East Kilbride, at 06:50 on Wednesday.
The man told the driver he had been been struck on the head and wanted to go to Hairmyres Hospital.
His condition deteriorated and was later transferred to the intensive care unit at the Queen Elizabeth University Hospital in Glasgow.
Police are keen to trace anyone who was travelling on the Airdrie to Hairmyres service.
Det Sgt Martin McKendrick said: "At the moment we still have to establish how, why and where the man came about his injuries.
"We know that there were two people, a man and a woman, at the bus stop in Westwood Road who got on at the same time as the man.
"We don't believe they were together or involved in any way with what happened but it's possible they may have seen or heard something or maybe got talking to the man prior to him getting on the bus."