When police investigate suspicious deaths, one of the key questions is: When did the victim die?

A study published Thursday in Science may lead forensics experts and detectives to a more precise answer in the future.

Researchers studying the microbes on decomposing bodies have found that the mix of bacteria and other organisms on dead bodies changes over time in a clear pattern.

“It’s very clocklike,” says Jessica Metcalf, a senior research associate at the University of Colorado and lead author of the study. “You have very predictable microbes showing up at very predictable times.”

Better pinpointing of the time of death could help narrow down a list of suspects by confirming or refuting alibis.

Investigators currently have several methods to estimate the time of death, including examining the stiffness of the body and the insects on the corpse, but these methods have shortcomings.

“Right now, every tool that a criminal investigator has is not perfect,” Metcalf says. “That means that people can get away with murder.”

In the hopes of finding a better way to pin down when someone died, Metcalf and her colleagues have been trying to harness the microbes that help bodies decompose. Shots profiled the work of Metcalf’s team two years ago as part of a series about the human microbiome.

First, the scientists placed four bodies in an open field at the Southeast Texas Applied Forensic Science Facility at Sam Houston State University in Huntsville, Texas, two in the winter and two the following spring. The field is secured by high fences and is used by various researchers for studies of decomposing bodies.

The team then repeatedly analyzed the bacteria, fungi and other microorganisms on the bodies and on the dirt beneath them for 143 and 82 days, respectively.

The researchers were able to narrow down the time of death to within two to four days within the first 25 days after death, regardless of the season.

“In a sense, your microbes are like witnesses to your death,” she says. “As you decompose, they can help investigators solve your murder.”

Combined with data collected from similar studies involving mice, the researchers also determined that investigators could identify unmarked graves by analyzing the microbes in dirt on the surface.

Other scientists welcomed the research as important for forensic science.

“It’s amazing,” says Jeffrey Tomberlin, a forensic entomologist at Texas A&M University. “They are definitely showing a lot of exciting data and potential applications in forensics as well as other areas.”

Metcalf and Tomberlin say investigators may be able to use microbes for more than just determining when someone died.

People leave behind traces of their microbes when they touch things, previous research has shown. So microbes could be used like fingerprints to determine whether someone handled a murder weapon or other objects at crime scenes, they say.

Scientists also hope to exploit the fact that people tend to unwittingly pick up the unique kinds of microbes that live in different places.

“We might be able to apply that information to determine where a person died,” Tomberlin says. “These are very critical questions for any forensic investigation.”

Microbes possibly could even be used to trace the movements of suspected terrorists, Tomberlin says.

“If you’re curious if a person is moving between borders, say Pakistan and Afghanistan, could you look at their microbial community?” he says. “That may be a possibility.”

Copyright 2015 NPR. To see more, visit http://www.npr.org/.

Transcript :

AUDIE CORNISH, HOST:

Forensic investigators look at all kinds of factors when trying to estimate exactly when a victim has died. For example, the rate of body decomposition, how much carbon and nitrogen has leaked into nearby soil. But what about bacteria? How could understanding those microorganisms help police in a murder investigation? That’s the question that NPR’s Rob Stein started to explore two years ago when he visited an unusual research laboratory in Texas known as a body farm.

(SOUNDBITE OF ARCHIVED BROADCAST)

ROB STEIN, BYLINE: It’s a strange scene. For a moment, you don’t even notice, scattered among the towering pine trees, the wild grass and weeds are bodies.

CORNISH: The farm is a place where forensic scientists study how bodies decompose. Next, Rob reports the results of that research, now published in the journal Science.

STEIN: This story feels like it could come right out of an episode of “Law & Order.”

(SOUNDBITE OF MIKE POST SONG, “LAW & ORDER THEME SONG”)

STEIN: When the cops find a murder victim’s body rotting in the woods, one key question is…

(SOUNDBITE OF “LAW & ORDER” TRANSITION TONES)

JESSICA METCALF: When did the victim die?

STEIN: Jessica Metcalf’s a forensic researcher at the University of Colorado. She says right now, medical examiners can only guesstimate based on things like how stiff the corpse is and what bugs are on it, but that doesn’t always work.

METCALF: Every tool that a criminal investigator has is not perfect. That means that people can get away with murder.

STEIN: Because the time of death is crucial for all sorts of things, like knocking down a suspect’s alibi.

METCALF: When a criminal investigator, say, is trying to solve a suspicious death and they want to know if particular people may have been involved, they need to know a timeframe.

STEIN: So Metcalf’s team decided to see if the microbes that live all over our bodies might help. They placed two bodies in a field at the lab in Texas in the winter and again in the spring and came back over and over again for weeks to sample the bacteria, yeast and other organisms that were helping the corpses decompose.

METCALF: If microbial communities decompose carcasses in a very predictable way, then we can use this as a tool to help estimate that time since death – that succession of microbes becomes like a microbial clock.

(SOUNDBITE OF “LAW & ORDER” TRANSITION TONES)

STEIN: And that’s exactly what they found. They could narrow down the time of death to within a couple of days, no matter what the season.

METCALF: It worked very well. You have very predictable microbes showing up at very predictable times. What we’re showing is that it’s very clock-like.

STEIN: Which makes it a very powerful tool for forensic science.

METCALF: In a sense, your microbes are like witnesses to your death. As you decompose, they can help investigators solve your murder.

(SOUNDBITE OF “LAW & ORDER” TRANSITION TONES)

STEIN: And other researchers agree.

JEFFREY TOMBERLIN: Oh, it’s amazing.

STEIN: Jeffrey Tomberlin is a forensic researcher at Texas A&M University. He says investigators may be able to use microbes for a lot more than just determining when someone died. They could analyze the microbes in soil to find unmarked graves. And it turns out, people tend to pick up the unique kinds of microbes that live in different places.

TOMBERLIN: We might be able to apply that information to determine where a person died and what they were doing before they died. And these are very critical questions in any forensic investigation.

STEIN: Researchers have already shown that people leave traces of their microbes behind when they touch things, so they could be used like fingerprints to determine who touched the murder weapon.

(SOUNDBITE OF “LAW & ORDER” TRANSITION TONES)

STEIN: And that’s not all – Tomberlin thinks microbes could be used to trace the movements of suspected terrorists.

TOMBERLIN: So if you’re curious if a person is moving between borders – say, Pakistan and Afghanistan – could you look at their microbial community? That may be a possibility.

STEIN: But, he says, this is all still really early. It’ll take a lot more research to know for sure when investigators can start to use our microbes as our witnesses. Rob Stein, NPR News.

(SOUNDBITE OF MIKE POST SONG, “LAW & ORDER THEME SONG”) Transcript provided by NPR, Copyright NPR.