Oh! *lightbulb over my head* I had already posted the comments below when I had a thought:
Do you mean how do "we" know it was BoA as in
we the public?
From that understanding, I now see what you mean!
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I'm guessing, so don't hesitate to set me straight if need be
It's probable that LE had already spoken to the AIW employee who told them his part in the hours leading up to the murders. When JW arrived at the fire, he spoke to LE who had possibly already determined he should go with them for an interview because of AIW employee's information. That would also be one way of knowing that JW was lying about some things so they kept pressing him.
About a fire or murder at someone's home: I don't know the procedure, but I think someone from fire dept. (or LE when determined a crime) immediately starts trying to contact a homeowner (and curious/impressed as to how they get that info quickly!) and at some point contacts their job or business.
In my neighborhood, they'd probably ask any neighbors for info, I have no clue how it would be handled there.