Whodunit?

The oil seemed to materialize out of nowhere in Chelsea's Island End River in the early morning hours of Jan. 10.

By day's end a small greenish film had expanded to several hundred square yards of viscous diesel fuel. As cleanup crews positioned booms around the estimated 10,000-gallon spill and began vacuuming it up, Coast Guard investigators set out to find its source.

Two weeks later they got an answer: The oil appeared to be a perfect match with fuel from a nearby ExxonMobil pipeline.

In a real-life version of the television drama ''CSI," a chemist at the Coast Guard's Marine Safety Laboratory in Groton, Conn., had painstakingly traced the same unique mix of molecules in the two samples, chemical ''fingerprints" that didn't exist in three other possible sources nearby. That, along with other evidence, pointed to ExxonMobil.

''There appears to be no other viable source near that spill," said Coast Guard Lieutenant Commander Claudia Gelzer. ExxonMobil denies that the oil is definitively theirs, but the federal government is going after the company to pay the $140,000 it spent to scrub the environment clean.

Oil has a life history, researchers say, that can be read like a book, revealing chemical characteristics that can document its origins, processing, and sometimes even the last tank it was pumped from.

''It's not like saying you have fuel oil in a tank and fuel oil in the environment and they are the same," said Kristy Juaire, a chemist at the Marine Safety Laboratory, which handles all the agency's oil fingerprinting. Oil, she said, ''is very particular."

Every year, Coast Guard officials attempt to fingerprint the oil in hundreds of unclaimed spills around the country to trace leaks, plug them, and make sure the culprits pay for cleanup. The instruments they use have helped identify everything from chemical contaminants in soil to drugs in urine but hold particular importance in oil spills, where other evidence is often submerged or washed away. Tracing a spill's source quickly can also help stop an undetected leak before it causes severe environmental damage.

Even when a spill's source is known, fingerprinting is vital to rule out what Coast Guard officials call ''spills of opportunity," when other boats dump their oily waste in the middle of a spill to save themselves the cost of proper disposal.

Near San Francisco over the course of the 1990s, a mysterious oil source killed some 51,000 seabirds. Thick oil and tar balls periodically washed up on shore, but scientists were at a loss to explain where it came from.

It wasn't until 2002 that authorities realized the sunken SS Jacob Luckenbach, a freighter loaded with 457,000 gallons of bunker fuel, might be to blame. The vessel collided with its sister ship in 1953 and sank about 17 miles off San Francisco. In 2002, authorities matched the oil on the sunken ship with the oil that had washed up, and the leak was plugged.

Many of the Coast Guard's biggest civil and criminal cases are decided in Juaire's lab, a nondescript basement research facility next to Long Island Sound. Samples from the Exxon Valdez spill are kept in a locked refrigerator there, along with samples of a 2003 oil spill in Buzzards Bay that leaked almost 100,000 gallons of fuel oil off Cape Cod.

Coast Guard officials say the lab's findings have consistently held up in court over the lab's nearly 30-year history.

To determine whether the samples match, Juaire, 27, feeds the oil samples into a gas chromatograph that separates each sample into its constituent parts. Then the samples are fed into a mass spectrometer that further reduces the sample into molecular fragments, revealing ''bio-markers" and other compounds that act as each sample's fingerprint.

This spectrometer information is spit out in reams of graphs with peaks and valleys that distinguish about 37 different molecular signatures.

Juaire places the graphs on a light table and compares each peak and valley among the samples. The exercise can take an entire day, and if the molecular signatures match, there is little doubt they derived from a common chemical source.

But it's never clear that a match will be found. Sometimes a sample degrades so much from sunlight or time in the water that its key chemical signatures are altered. Other times investigators aren't able to pinpoint the true origin of a spill or conclude there may be more than one source with the same fingerprint.

For example, Juaire says, there have been cases where fishing vessels were all fueled from the same dock, making it tough to determine which vessel was to blame for a spill.

In fact, Juaire's lab makes a positive match only half the time. In the last four years, there have been 152 oil spills of more than 100 gallons each that haven't been solved.

''I'd rather be cautious and know someone got off than be wrong," said Juaire, who took the chemist's job after graduating with a master's degree in geochemistry from Brown University five years ago.

Before 9/11, the laboratory typically processed about 400 cases each year, representing thousands of samples. But the number plummeted when the Coast Guard took on more homeland defense duties.

Now the number of samples is increasing again, and last year the lab was involved in 256 cases.

In Chelsea, ExxonMobil says the Coast Guard has not proved the oil spill is theirs. A spokesman said the company always accepts responsibility when it causes a spill, but in this case ''we don't believe their evidence is conclusive."

''Diesel fuel is a very standardized product and can be found on vessels and shore facilities throughout the Boston Harbor area," said the spokesman, Brian Dunphy.

Coast Guard officials said they have other evidence linking the spill to the company but declined to comment in depth because their investigation is ongoing.

''In CSI, they can solve the crime in nine seconds with the science and it's 100 percent certain," said Christopher M. Reddy, an associate scientist at Woods Hole Oceanographic Institution who studies marine pollution. ''But in a lot of these cases there is a certain level of uncertainty and it may take time for the most accurate results." Exxon, he said, is sure to push the Coast Guard to prove its case.

Beth Daley can be reached by e-mail at bdaley@globe.com.