You stated two criteria. You assess whether there is a reasonable prospect of conviction and whether an offence has been committed. As a basic principle, it is established that the individual has committed offences for which it is possible to secure a conviction and it is determined where the public interest lies. Those are your criteria.
But if the case involves a person who has previously belonged to the political system, how independent are you? I'm talking about situations in which all conditions are met, of course.