So how do investigators determine the timeline of a person's death and identify the remains?
Scientists can track how a body decomposes — which tissues break down first, how insects go to work — but how long it takes for a person's body to go from complete to skeletal depends on several variables.
And, similarly, how long it takes to match skeletal remains to a person is also dependent on multiple variables.
"It's predictable in the stages that will occur," said Corey Ragsdale, a professor in anthropology at Southern Illinois University Edwardsville. Those stages include chemical breakdown and the order in which tissues break down.
"But the actual timing of it is really, really flexible," Ragsdale said.
Skeletal remains were found in Litchfield on Wednesday evening, and a month earlier a man's decomposing remains were found in Belleville.
On Thursday, police said they had not determined the identity of the remains in Litchfield, but they said the remains do not appear to be those of a Macoupin County woman who has been missing since January.
Authorities in Belleville have not released any new information on the body found March 25, other than it belonged to an older adult with a light complexion.
Ragsdale, an SIUE professor who specializes in forensic anthropology, said weather conditions, insect activity and body size all play into how long it takes for a body to decompose.
"But in all cases, there's a huge amount of information we can get from a human skeleton," he said. "Even just fragments of a skeleton are pretty amazing."
What scientists do with forensic cases is to build a "biological profile," Ragsdale said.
That includes stature or height of the person, the estimated birth sex, an age estimate that can be accurate within five years, and what they call "ancestry," or if the person is from European, Asian or African ancestry.
"We can get a whole bunch of info about their life and their health, trying to create a profile," he said.
That profile is usually given to law enforcement, who see if it matches anyone known to be missing.
"Skeletal cases have gone years without getting solved, but I think we’re getting past that. We’re getting more information from fewer things," Ragsdale said.
He said decomposition of a body to skeletal form would be "reasonable" in a few months given conditions in the area this spring.
"Three months is reasonable. In the cold; some bodies (would take) up to five to six months."
A professor at the University of Tennessee, known for its work with The Body Farm, focuses on the microbial actions that take place in decomposition.
The fastest decomposition to skeletal form she has seen was three weeks in summer, said Jennifer DeBruyn, an associate professor at the University of Tennessee.
"In winter it could take four to six months or longer," DeBruyn said. "Going from a living, breathing human to a skeleton involves a couple of key processes."
She said it starts with death where cells lose oxygen and "kick into suicide mode."
The cells start breaking down into their bigger components. Then the gut microbes, which had been fed by digestion, eats those broken-down cell components. That process usually involves the production of stinky gases that usually causes bloating, she said.
Then the insects get to work, and any scavenging animals also make their contributions.
How long all that takes depends a lot on the temperature.
"It's a simple chemistry law... hotter is faster. On a hot summer day we could go from fresh body to a bloated body in a matter of days. In the winter, it could take weeks to get to that stage."
"Based on a cooler (than usual) spring, it seems fast to be completely skeletonized in three months," DeBruyn said. "When it's below 50, we don't see a lot of flying insect activity. ... We might expect to see a lot of scavenger activity though."