Excellent info on decomposing body
http://www.trutv.com/library/crime/criminal_mind/forensics/bill_bass/4.html
"There are a lot of factors that can affect how a body decomposes, but we found that the major two are climate and insects. When a person dies, the body begins to decay immediately and the enzymes in the digestive system begin to eat the tissue. You putrefy, and this gives off a smell. Up until about two and a half weeks, one of the best ways to tell how long a body is dead is to look at the insect activity. The first of the critters to be attracted to a decaying body are the blowflies. They come along and lay their eggs, which hatch into maggots. The maggots then eat the decaying tissue in a fairly predictable way." Measuring and recording this information, along with climatic variables such as temperature and humidity, gave the facility its raison d'être.
a scientist can determine how long a body was lying in a particular spot, or whether it was placed somewhere and then movedand when that occurred.