Apparently her killer fascination stemmed from a 1984 murder near her Illinois home when she was fourteen.
She walked to the abandoned crime scene a few days later and collected the victim's broken Walkman.
But one suspects the roots go deeper than that.
Indeed it's McNamara's own story - glimpsed in tantalising fragments - which is the unresolved mystery here; a fraught home life, a rebellious teen, that increasing fascination with the darkest sides of human nature, which led to her setting up the truecrimediary.com website before narrowing her focus to the Golden State Killer.
McNamara was well aware of her obsession's toll on her health and family but, like an addict couldn't stop, even as many of her suspects in IBGITD became dead ends - "...the monsters recede but never vanish. They are long dead and being born as I write".
No doubt the dramatic possibilities of this dual mystery played a part in HBO recently picking up the rights to make a docu series.
Post arrest - it's clear McNamara was on the right track - her hunch that the perp had military and/or police training was correct, as was her belief that science would catch him in the end.
The tragedy is that she didn't live to see it.
I'll Be Gone in the Dark: One Woman's Obsessive Search for the Golden State Killer.
Michelle McNamara
(Faber & Faber $32.99)