A man in his late teens is in a critical condition in hospital after he fell through the glass verandah roof on the Work and Income building in Watt St on Friday night.
Whanganui police, called around 8.30pm, found him lying on the concrete at the building's entrance, surrounded by shattered glass.
Sergeant Kevin Smith said police were trying to find out why the youth was on the building.
Another investigation under way is into the cause of a three-car crash before 4pm on Saturday afternoon at the Victoria Ave/Glasgow St traffic lights.
Mr Smith said police were halfway through speaking to witnesses to the crash, which appeared to have occurred when one driver did not stop for a red light. The vehicle hit another and shunted it into a vehicle waiting at the lights.
No one was injured, and charges were likely, Mr Smith said.
Mid-afternoon on Saturday, a vehicle's tyres were shredded when it hit kerbing then crashed into an armco roadside barrier south of Kai Iwi township.
Police were investigating the cause of the accident. Mr Smith said speed was not a factor.