SOUTH CAROLINA -- Three deaths caused by ice cream tainted by bacteria last spring were part of an outbreak that had been going on for years.
In April, Blue Bell Creameries recalled all of its products in 23 states. The ice cream was contaminated with listeria which can be fatal to people who are ill or have compromised immune systems.
In part two of the investigation, investigators show CBS News how the case of the mystery deaths was solved.
When Megan Davis and her team from the South Carolina Department of Health randomly sampled ten products from a local Blue Bell distribution center in January -- the last thing they expected to find was listeria.
"It was unbelievable actually," said Davis. "We never in a million years thought we'd find a positive sample."
Two of the ten samples tested positive, but just to be sure, they went back and collected 30 more.
"All 30 of the samples that we tested, tested positive for listeria," said Davis. "Yes, stunning. A little scary that those products were going to consumers."
Davis uploaded their findings into Pulsenet, a database of DNA fingerprints the Center for Disease Control monitors to identity outbreaks nationwide.
"The listeria germs found in South Carolina in the ice cream matched illnesses in a hospital in Kansas," said Dr. Robert Tauxe, deputy director of CDC's Foodborne Disease Division.
That hospital was Via Christi St. Francis in Wichita. Listeria had sickened five of their patients over the past year, but the hospital couldn't figure out where it was coming from. The listeria patterns found in South Carolina solved the mystery in Kansas.
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Turns out, all five of the patients had been served milkshakes made with Blue Bell ice cream.
In mid-February, Blue Bell quietly pulled all the ice cream made on the machine that had produced the ice cream testing positive in South Carolina, citing a quality issue. But Via Christi still had plenty of other Blue Bell products in its freezers. The Kansas Department of Health tested 45 of them, and found another hit.
"When that was loaded up into the Pulsenet database, it matched five other patients, but these weren't recent," said Dr. Tauxe.
These five cases came from three different states, going back to 2010.
"Unknown and unappreciated to anyone, a low level outbreak was going on four or five years," said Dr. Tauxe.
An outbreak no one was looking for, that very nearly went undetected.
"Our inspector could have picked two different ice cream products to test," said Davis. "What if he hadn't picked those two samples? It may have had a different outcome."
Pulsenet enabled the CDC to trace the tainted ice cream, not just the Blue Bell's main plant in Brenham, Texas, but to another Blue Bell factory in Broken Arrow, Oklahoma, as well. Blue Bell has said it is increasing its focus on sanitation and cleaning and is being evaluated by independent microbiologists.
In a statement today to CBS News, Blue Bell said: "While we do not usually comment on matters involving current or former company employees, the isolated views expressed by two former Blue Bell employees on CBS News do not reflect the experience of the vast majority of our employees, who know we take the cleanliness of our facilities and the quality of our products very seriously. Over the years, we have welcomed an average of 200,000 visitors a year to tour our Brenham plant and see our operation for themselves. Our employees, many of whom have worked with us for 20 or 30 years or more, are hardworking, dedicated individuals who are committed to producing the best ice cream possible. Our top priority and commitment is to produce high quality, safe, delicious ice cream for our customers."