The Caravan Crime

A man, who will turn out to have a murky past as a blackmailer, is found dead in a carriage; the coachman says that someone helped the man in an obvious state of drunkenness into the carriage, and then rode with him giving the driver an exact address, a sign that he knew the victim well; but on arrival the drunkard is found dead, killed with chloroform, and there is no trace of the mysterious passenger. Who didit?