State police in Greensburg are investigating the murder of a man early yesterday afternoon at a mobile home near Torrance in Derry Township.  The victim was in his early twenties and police are not releasing details on the crime as yet.  The name of the victim is being withheld pending notification of his family, and investigators are also not releasing the manner of death, but say they are certain it was murder and they have begun a search for a known suspect.  The man was slain at a trailer along Stone Quarry Road.