When RA was interviewed prior to his arrest, LE did not know how he'd respond, whether he'd respond, whether he'd implicate anyone else. IMO LE has the authority to detain/arrest a POI at which time they have cause.
Had RA for example implicated others in a credible way, LE may have pivoted, leaning hard on RA to get to others who wielded the knife/blade/hook/tool/cutter, in which case (if it were true), RA might have ultimately gotten a sweetheart deal in exchange for testifying against the others.
It was reasonable to talk to him, to see how he responded to lines of questioning, crime scene photos if shown, etc.
IMO RA's answers which he supplied voluntarily showed deceit where it mattered most. Akin to an answer for everything even if it doesn't fit the evidence LE had at hand.
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I don't know this to be true but I can IMAGINE something like this-- a map, showing the timestamps and key individuals. Showing a dot for where and when LE could place him. Then: if you saw this group of juveniles and they saw you, that's at such-and-such time... and if BB saw you, turned, then passed Abby and Libby, how did you not see them?
Was absorbed in the stock ticker on my phone.
Where is the phone that will confirm that?
Oh, that's right, I left before that.
How long were you on the platform? Where did you go from there?
That's how interviews go, the ones I've seen played in trials. Lots of questions, lots of answers, lots of opportunities for a liar to get tripped up. Which IMO is exactly what happened.
JMO