That is true. It is best to consider every angle concerning the suspects. I can understand why he would seem like an unlikely candidate, however...

[ He thinks it is way too early to throw Xerxes out as a viable suspect, at least, even if he is not sure who it is. ]

... I also do not think it wise to discount him entirely. It could prove to be a mistake. Hopefully we should be able to glean some more tomorrow.