who is suspected of the crime commited here, and why?
The initial general response appears to be that the culprit must be one of two men. After all the evidence clearly left behind was examined by people from "the Hulks" and two convicts had escaped from the aforesaid prison ship.
This general suspicion is then narrowed down to one man. As two convicts had escaped and the one who had been retaken still possessed his leg iron it was believed that the convict's leg iron left at the scene must have belonged to the second convict - Magwitch.
This belief is narrowed further to the specific suspicions of Pip - that the leg iron belonged to " his convict" but that it was not his convict - Magwitch - who had done this crime.
This brings us to the reasoning for Pip's belief. Pip believed that either Orlick or the "strange man" Pip had seen must be the culprit (yes reader we know the strange man was the other convict and the first paragraph above reminds us how it could not have been him) . The reader at this point will know that it could not have been the "strange man" so we are left thinking perhaps it was Orlick and we listen further to Pip's reasoning.
Pip therefore continues to reason that the perpetrator of this crime could have been Orlick. Orlick had quarrelled previously with Pip's sister.
Amongst this is the childhood fear that makes Pip consider that he has some responsibility in this crime because of his silence about the convict, that he had "provided the weapon" .
Not suprisingly perhaps we also discover later in the chapter that all along the officers had suspected poor Joe. There is reference to the things he is allowed to do now that Pip's sister is incapacitated to some extent. There is also reference to tools that as a blacksmith he would be very familiar with. We are also informed that Joe was the person who found Pip's sister, nothing had been taken and prior to his return home nobody had seen her except for a farm labourer who could only swear to having seen her alive an hour before Joe's discovery. All of this evidence suggested Joe may have done the deed and perhaps Joe's change in life style could have been a reason for this.
We end the chapter knowing no more about who it actually was who committed the crime and so have to side with Pip's comment about his sister's treatment of Orlick as a comment on who at this stage might have done the deed and why -we know "no more than I did what to make of it."