The hunt by Colorado scientists’ to identify the source of a national listeria outbreak that has been linked to eight deaths involved two weeks of brainstorming, studying blood samples and confiscating half-eaten food from patients’ refrigerators.

Health officials also shopped for grocery-store cantaloupe before pinpointing a single farm in Holly as the source.

State epidemiologist Alicia Cronquist thought it slightly odd when she learned two people were sickened by listeria bacteria within days.

When two more reports arrived at the state health department in late August, Cronquist figured she was dealing with an outbreak.

In the early stages of Colorado’s investigation, county health authorities used 15-page questionnaires to interview patients and their families about what they ate and where they bought it.

"The vast majority of people are elderly," Cronquist said. "The average age is in the 80s and they are quite ill. Their family members are at their bedside, and we are asking them to remember food that they ate a month ago. They are actually very difficult interviews."

Meanwhile, the bacteria in patients’ blood was isolated, and state microbiologist Hugh Maguire’s labs deconstructed the DNA profiles to see whether they had listeria strains in common.

Those profiles were uploaded into a CDC database of data from across the nation.

Scientists pulled food from patients’ homes, including typical listeria suspects like deli meats, hot dogs and dairy products.

By Sept. 2, the state food lab determined that two patients had matching strains and two other patients matched in a separate strain. With hundreds of strains of listeria, two or more can be on the same food.

Epidemiologists in patients’ home counties interviewed the patients to look for patterns while the state health department faxed and emailed a listeria alert to doctors, hospitals and labs.

When Cronquist checked a CDC database tracking foods that listeria victims reported eating, she found that all the patients she was tracking had eaten cantaloupe.

Health authorities purchased 15 cantaloupes at three grocery stores and tested the rind and flesh for listeria bacteria. They also were testing patients’ leftover melon. Maguire’s lab fast-tracked the genetic matching, setting aside some of the lab’s other, more routine work.

A week after the first public warning, health authorities announced they had linked the source of the poisoning to cantaloupe.

As more patient blood samples arrived at the state lab, they fell into three distinct strains. Cantaloupe taken from patients’ refrigerators had the same strains but no sticker naming the farm. In interviews, though, patients volunteered that the cantaloupe said ‘Rocky Ford’ on it or was extra sweet.

By tracking the melon purchases of patients back to the distribution trucks, investigators from the state and the Food and Drug Administration narrowed the focus to two farms and sampled soil and machinery.

Two days after warning people not to eat Rocky Ford cantaloupe, health officials announced they had pinpointed the farm.

Jensen Farms in Holly recalled its cantaloupes Sept. 14, while farmers in the Rocky Ford region miles away lamented how their produce was swept into early warnings about cantaloupe.

Meanwhile, 7NEWS in Denver has confirmed the investigation is expanding beyond Jensen Farms and that a company that sprays treated human waste – biosolids — confirmed to 7NEWS it has been contacted by investigators. State investigators confirmed they want to know if biosolids may have caused the contamination.

Parker Ag Services vice president Mike Shearp told 7NEWS that government investigators have questioned him in recent days as to where those biosolids were applied. He said the substance was applied to a field directly across from a Jensen Farms field years ago.

Shearp maintained that the contamination will not be traced back to his operation.

Colorado State University animal science professor Lawrence Goodridge said, "If processed properly, there should not be pathogens. If they are not processed properly, if the wastewater treatment process breaks down, they could be (a) source of pathogenic bacteria such as salmonella, listeria and other pathogens."

A spokeswoman with Jensen Farms said the company does not use biosolids in its operation.