I don't think it is reasonable to hear that a girl has committed a crime and instantly start asking if she is transgender.
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I think it is clear that this speculation comes froms a place of wanting this crime to fit a narrative.
If that narrative is "trans people are more likely to commit violent crime", then I can only speak for myself, but I feel like that's a fundamental misunderstanding on the part of people who subscribe to gender identity ideology in terms of what people with gender critical views believe.
When I ask whether someone reported as being female is also trans, I'm doing so because the assumption that I'm checking these events against is that "males are far more likely to commit violent crime". It's got nothing to do with trans people other than trying to determine the actual sex of the perpetrator. Obviously, if they aren't female and trans, then this doesn't fit this assumption about male violence being more common, and so this already shocking crime appears even more shocking as it's far more unusual for this kind of crime to be committed by a female. It's hard to make sense of a child killing people and to make sense of innocent children being killed, so any fact that might it make a tiny bit more sense is understandably sought after.
Because I don't subscribe to gender identity being anything more than a personal belief (which anyone is entitled to hold and express, like any other reasonable religious/philosophical belief within the usual parameters), I don't view trans people as a distinct class of people in terms of how their behaviour might differ from others'.
Sure, there might be subgroups of trans-identifying people on hormones or who haven't been able to access routine MH care who might be more at risk of certain behaviours. But the question people have been asking is about whether they were trans specifically, not about their medical or MH history.
While I can see why trans people see themselves as a separate category of people distinct from the group they would call 'cis' people, this distinction is only meaningful to people who subscribe to gender identity ideology. Therefore, given that I don't personally believe in that distinction, why would I view a trans person's actions through the lens of a belief system I don't believe in?