Colony feral found dead

I care for a feral colony at work. The colony includes about 28 cats,
all trapped/tested/vaccinated and fixed.

Today I found one of my brood in the parking lot, dead. She was a true
feral, never a bit socialized except knowing who brought the food and
where he put it. I'd known her for over three years, since she was a
kitten. Unfortunately, she was always cagey and smart, avoiding capture
until just last winter. She was the one responsible for many of the
litters I dealt with before I finally managed--with a lot of help--to
control the colony population. She was among the last of the colony to
be trapped and fixed. Sometimes I think she actually gave herself up
because she was tired of all the kittens.

Anyway, she's gone. I looked her over carefully before disposing of her
carcass. She had no cuts, no scrapes. No puncture wounds of any sort. No
blood was anywhere in the area (she was in a vacant parking space at the
far end of the lot near some thick landscaping). Her mouth and her eyes
were open, the nictating membranes about half-closed. She had no broken
bones, from what I could tell (she was in rigor mortis). In fact, she
looked perfectly healthy.

What do you think are the chances that someone poisoned her? What other
causes could there be?

I cannot afford necropsies on these animals, but if there is someone out
at the colony with poison, I would like to know about it and maybe do
something to prevent it. Poisoning animals is a crime in this state.
Inspired by TV shows like CSI and movies I've seen, I've decided to look
into veterinary forensics. Using that as a search term on Google, I came
up right away with this site:

http://www.vet.uga.edu/vpp/IA/SRP/vfp/

Does anyone have any printed texts they would recommend, suitable for
the educated layman, on investigating animal cruelty?

TIA.