They have some evidence that he downloaded child porn, but it's the trial and jury that will evaluate if it's sufficient beyond all reasonable doubt. If they believe that current evidence has some chance of being not sufficient and there's extra incriminating evidence, then it's their right and duty to obtain that before passing it on to the court.

You can't answer the question "if they can prove" before court, as it's decided only then. You must finish the evidence gathering before you have a judgement on that.