There is already a pretty good book on the subject, from 1980, by Thomas Maeder.
http://www.amazon.com/Unspeakable-Crimes-Petiot-Thomas-Maeder/dp/0316543667
It’s been a few years since I read it, but as I recall one of the biggest problems with the case was that there was quite a bit of evidence that Petoit did not act alone. But the circumstances, newly liberated France, resulted in a pretty shoddy investigation.
But the circumstances, newly liberated France, resulted in a pretty shoddy investigation.
Yeah, this guy was smart and took advantage of the chaotic circumstances. The Wikipedia entry names three accomplices: Marcel_Petiot