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In a small, obscure government laboratory, two self-described nerds are doing microscopic detective work, trying to determine the origins of the major narcotics shipments that U.S. agents intercept at the border.

Andrew Laurence, 33, and Shannon Ferguson, 32, are forensic palynologists who specialize in the identification of pollen grains. They can extract the particles from almost anything — drug packages, clothing, the air filters of impounded vehicles — and use that information to determine where an item might have traveled. Pollen grains stick to objects in abundance, and the precise combination of those particles creates what forensic palynologists call a “signal,” not unlike a fingerprint.

“All plants have a unique pollen grain,” said Laurence, who was hired by U.S. Customs and Border Protection in 2012 to establish the lab in Chicago. “So every part of the world is going to have a different pollen print.”

They are at the forefront of helping officials determine, for example, whether shipments of deadly fentanyl were made in Mexico or originated in China.

The technique also has applications for counterterrorism and other national security purposes, Laurence said, such as testing material found on explosives or criminal suspects. But he said he couldn’t talk about that part.

Border Patrol agents also have been sending in soil samples from the field, allowing Laurence and Ferguson to build detailed pollen maps of the U.S.-Mexico border that might help identify where shipments crossed.

The information is not used for prosecution, but it can help substantiate or cast doubt on information collected in interviews or from informants.

Laurence said most of the samples he is asked to identify now are fentanyl. Wearing hazmat suites and respirator masks, he and Ferguson will sometimes dissolve an entire kilo brick in hydrofluoric acid and other chemicals, then separate out pollen grains using centrifuges.

After they separate the pollen on their samples, they inject dye so they can view the structure under microscopes. Many of the grains are familiar, such as pine and oak that are scattered across the planet. The researchers are looking for the percentages of those species in combination with more distinctive grains.

The in-house reference library has about 6,000 pollen types, and Laurence and Ferguson have access to another 30,000 at the Field Museum of Natural History. Worldwide, there are about 380,000 types of pollen grain, Laurence said. About 70% of what they find is composed of grain types in their reference library, but Laurence said there are many parts of the world that remain “black holes,” where little mapping has been done.

The palynologists also assist state and local authorities on hard-to-solve homicide investigations, including cold cases. Pollen molecules are extremely durable, so the grains remain intact on decades-old crime-scene evidence. “Nature’s plastic,” Ferguson said. “You can’t get rid of it.”

“Pollen itself is best described as a flying fossil,” Laurence said.

In 2015, Laurence and Ferguson helped crack the high-profile Bella Bond murder case after the body of a toddler was found in Boston Harbor. Police sent Laurence garments and hair samples, and he was able to identify grains from two types of rare cedar trees that were in a Boston-area arboretum.

The discovery helped police narrow their search, leading them to arrest the child’s mother and her boyfriend, who lived near the arboretum. The boyfriend was convicted of second-degree murder, and the mother was convicted of being an accessory after the fact.

More recently, Laurence and Ferguson worked on a 37-year-old homicide case from Ohio, identifying pollen from the crime scene, and the victim was finally identified in combination with DNA testing.

A relatively abstruse area of research, palynology produces few graduates. Laurence and Ferguson said most of the Ph.D. candidates are immediately hired by oil companies to test organic material from core samples to determine the geological strata, so engineers can direct crews to go deeper or pull back. The palynologists working for Exxon or Shell earn many times more than those in government labs. “Basically you have to be an idealist to work on the government side,” Laurence said.

They are trying to recruit more palynologists to the lab because their backlog of cases is 18 months to two years.

At Texas A&M, Laurence wrote his dissertation on “ancient starches,” analyzing material from earthen ovens to identify what grains were being cooked. His mentor, Vaughn Bryant, began promoting forensic palynology in the 1970s. But Bryant said he got little traction promoting the law enforcement potential until the Sept 11, 2001, terrorist attacks, then spent a decade analyzing samples for U.S. intelligence agencies.

“The next thing that happened was I got call from the Department of Homeland Security, and they said: ‘Do you think you could help us find out where this marijuana is coming from?’ ”