(CNN)It was a mystery by any definition of the word -- a rare tropical infection that had sickened people in the decidedly non-tropical states of Minnesota and Kansas, as well as Texas. The first patient to get sick, in Kansas in March, died.
They all were infected with a bacteria known as Burkholderia pseudomallei, and the disease it causes is called melioidosis, marked by non-specific symptoms such as cough and shortness of breath, weakness, fatigue and nausea. It's most commonly seen in Thailand, Malaysia, Singapore and Northern Australia, and is found in soil and contaminated water.
It's not expected in middle America, and certainly not in a lavender-scented aromatherapy spray with gemstones.
Usually, when Americans are diagnosed with melioidosis, it's associated with travel. But these cases popped up in the middle of a pandemic, when international travel was virtually nonexistent. And none of the affected families had traveled.
The US Centers for Disease Control and Prevention issued a heath alert last June, when the three cases in Kansas, Minnesota and Texas were linked. By the time they found the source this month, it had made four people sick and killed two of them.
The trail had gone cold in Kansas, said CDC epidemiologist Dr. Jennifer McQuiston, who helped lead the investigation. The CDC worked with the state health departments to try to figure out how the people had become infected with such an unusual bug.
"It really was a fishing expedition because we didn't have any early clues to guide us in any direction," McQuiston told CNN.
"The teams really looked at personal care products, lotions, soaps, food items, vitamins -- things they might have been exposed to," McQuiston said.
"Cleaning products -- all of those sorts of things. The thing about Burkholderia pseudomallei is it really needs a moist or wet environment in order to survive. It can survive in some types of moisture that you would not normally think of a bacteria surviving in, so even hand sanitizers."
Then in July, a patient died of melioidosis in Georgia. Genetic testing linked it to the other three cases.
The CDC teams doubled down on their searches, going through all the products they could find that might possibly be a source of the bacteria. Even so, there was no smoking gun.
"They had tested several hundred specimens and it looked like it was coming to a dead end," McQuiston said.
In a last-ditch effort, they went back to the home of the last patient for another look earlier this month.
"And in that particular second search, they collected a specimen from an air freshener bottle that had not been collected the first time around, and this week that we got a positive PCR results out of that air freshener bottle for Burkholderia pseudomallei," McQuiston said.
PCR -- polymerase chain reaction -- is the same type of lab test used to amplify genetic material for coronavirus tests. This time, it found genetic material from the bacterial crime suspect.
There, they found it: "Better Homes & Gardens Lavender & Chamomile Essential Oil Infused Aromatherapy Room Spray with Gemstones." The product had been manufactured in India and sold at Walmart.
Walmart recalled the product Friday.
"We were all so relieved to have something that pointed to a source of infection because our biggest worry was that whatever had caused infection in those four previous cases might still be out there posing a health risk to people," McQuiston said.
"This showed us that that was true -- our instincts on that were right because there are households in America that have this bottle of air fragrance in their homes, potentially spraying it," she added.
"That's why we thought it was so important to go out quickly with this information, even though we're still waiting on the sequence confirmation results showing that what came from the bottle matched the patient from Georgia. The PCR results are what prompted us to go forward with this announcement."
The CDC has been able to link the strain of bacteria to the patients in Texas, Kansas and Minnesota. "So we've got A connected to B, B connected to C and the sequence results will help us connect A to C," McQuiston said.
It's not clear which ingredient in the spray may be the contaminant. It might the "gemstones," however.
"Rocks are collected from the environment and there's bacteria in the environment, so if the rocks weren't sterilized before they went in, that's a possibility," McQuiston said. "The other thing is the possibility that another component was contaminated and the rocks made a little micro-environment in that bottle for the bacteria to grow in," she added. "So the significance of the rocks we don't really know yet, but having rocks in a fragrance bottle is definitely unusual. So that's something I think we're interested in looking at."
The same manufacturer made other scents using "gemstones" that the CDC will be examining, McQuiston said.
It's also not clear how people might get infected by a spray. It doesn't necessarily appear as if the victims breathed it in.
"A lot of people said that they sprayed this on their pillows at night before they went to sleep, to give them a nice fragrance -- so you can imagine that there are, there are uses of this, even beyond just spraying in a room, where it might bring someone in very close contact with the bacteria," McQuiston said.
Now investigators will go back to see if the patient in Texas might have purchased the same brand of spray.
"There was not a mention of this specific product or brand in the initial interview questions I think that the states undertook with those families," McQuiston said. "I believe there was a mention of a possible fragrance room spray in a family member of the Texas patient. So I think we're going to try to go back and drill down a little more carefully."
This is the difficult part.
"We may or may not ever make that connection, given that we're several months out now. The bottle may not be in the home any more but I think there's going to be an attempt made." It likely will not be possible for the Kansas patient, who died in March, or for the Minnesota patient, she said.
"But I will say we've heard that both of those individuals had a history of using scented products or essential oil type products so I think that you can imagine the possibility of that link is there."
CDC epidemiologists are often called disease detectives, and this is an example of why.
"You weigh the excitement of being able to put the pieces of this puzzle together with, really, the horrific knowledge that two people died and four families were dramatically impacted by this," McQuiston said. "And I really think the knowledge of how serious this was is what drove our scientists to work so hard to try to solve this mystery."