An investigation revealed that the victim, who genetic tests identified as a Zapotec Indian, was a petite woman age 30 to 40 who had been buried in a shallow grave in Mammoth Lakes National Forest, probably the preceding autumn. But the case stumped police for years before the woman's hair yielded important clues.

A new forensic technique, developed by University of Utah scientists, analyzes chemical isotopes in human hair to offer insight that may help identify the victim.

The woman's hair revealed she had likely spent the last few years of her life in the southwestern U.S. or northern Mexico, according to Jim Ehleringer, a U. biology professor.

His technique is possible because the hydrogen and oxygen isotopes in water people drink are replicated in hair, bones and teeth, and can therefore be pegged to the geography, according to findings published this week in Proceedings of the National Academy of Sciences by Ehleringer and Thure Cerling, a U. professor of geology and geophysics.

"The old adage appears to be true, that you are what you eat," said Ehleringer, who co-founded the business IsoForensics to develop crime-fighting uses for his technology. "When you think of the hydrogen and oxygen in your hair, it can only come from a couple of places. A significant portion comes from what you're drinking."

If successful, IsoForensics' technique could be a valuable tool in identifying Jane and John Does and solving cold cases in which the victims' identities are known. Detectives used the technique in two Salt Lake County cold cases, both involving bodies found dumped along the Great Salt Lake's south shore.

"It's a phenomenal technique," said sheriff's Detective Todd Park, a cold case specialist. "In the realm of unidentified victims, this technology narrows down the geographical location of places to look. In cases like these, the world is your crime scene. With this technology, it's going to narrow significantly."

The key to the technique is the isotopic composition of municipal water. Isotopes are forms of an element with different atomic masses. An isotope of a particular element has an atomic nucleus whose protons match the element's atomic weight, but holds a different number of neutrons. The greater the number of neutrons, the "heavier" the isotope is said to be.

IsoForensics zeroes in on water's atomic components, hydrogen and oxygen, found in hair, and compares the ratio of heavy to light isotopes. Why does water vary isotopically according to geography?

Heavier water tends to precipitate out of clouds first, so the ratio of heavy to light isotopes declines as storm clouds move away from the points of origin. Imagine a raincloud as a glass of water.

"When rain falls in San Francisco, some of the water leaves the glass. The isotopes that fall out first are the heavier isotopes," Ehleringer explained. "When the cloud reaches the Sierra Nevada there is less heavy hydrogen and less heavy in oxygen, so the cloud is lighter isotopically. By the time you get to Salt Lake City you should have negative numbers."

The isotope sleuths look for isotopic variations in hair in the order of 1/10,000th percent, requiring precise measuring equipment, Ehleringer said. The research team has generated maps that graphically match isotopic makeup of a person's hair with regions.

Teeth and bone also reflect the water we consume, but over a much longer time frame than hair, which grows quickly and is shorn away.

"You and I have built into our bodies a map of where we've been for the past several years, where we were born and raised, where we have been in the last year," Ehleringer said.

Isotope analysis of the remains in the Mammoth case produced a time line of the woman's possible travel and diet that corresponded well with her ethnic identity as a Zapotec.

She was likely born and raised in the border regions between Mexico and the U.S. and lived on a poor, corn-rich diet, then moved to southern Mexico where she spent 10 years, according to information posted to the Doe Network. Sometime in the past two years of her life she traveled to California, where she was likely murdered near Mammoth Lakes.

bmaffly@sltrib.com