In other words. if some evidence (but not conclusive) exists of somebody else comitting a crime, can a defence lawyer use this when representing their client? For instance, in the Glen Sebastian Burns and Atif Ahmad Rafay case, where the two were accused of murdering the latter's parents, there was an FBI informant that had told the police that Islamic fundamentalists may have been responsible, although the police did not pursue this line of inquiry. Could a defence lawyer raise something like that in court?
The defense lawyer has the duty to do the best for his client. The client will be convicted if he or she is guilty beyond reasonable doubt. If the lawyer can create a reasonable doubt and manages to free his client then he has done a good job.
So yes, if the lawyer knows that some other person might have committed the crime, to the degree that it creates reasonable doubt, then the lawyer must raise this. Of course if it turns out that there is just some phantasist making wild accusations, that might not be helpful.