I found it in DOI, p. 123,ppr back:
Sitting in her car on the way to the doctor's office,frightening thoughts started racing through my mind.Almost as if JonBenet's death were occuring for the first time,the terror and horror of what had happened settled over me.My heart pounded like a bass drum booming away inside my chest.The "thump-thump" kept increasing and I fought to breathe.As if someone were strangling me,I desperately gasped for air.I kept thinking,I've lost my daughter,who killed my child?The fierce reality of it all came crashing down, and a panic attack swallowed me.
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To me,it sounds like she is remembering strangling JB ..as though she's projecting that onto herself. *That's what I think caused the panic attack.
I don't recall who,I'd have to search,but I remember a Dr. saying he'd never once heard anyone say a panic attack was like being strangled;it had never been described to him that way.
But,like everything else,it's all open to interpretation.