Medical investigators are trying to unravel a mystery behind a bacteria that’s caused blood infections in dozens of people in Wisconsin, including 17 people who have died.

More perplexing is that health officials confirmed this week that a Michigan resident died after contracting a bloodstream infection that matches the one detected in Wisconsin.

“Everybody is frustrated about not being able to identify the source on this one, and nobody more so than the investigating team,” said Chris Braden, deputy director of the national center for emerging and zoonotic infectious diseases at the Centers for Disease Control and Prevention. “With the increasing numbers of cases week after week, there’s no indication that it’s stopping.”

Since the outbreak began in November, 54 people in 12 southeastern Wisconsin counties have been sickened by bloodstream infections caused by Elizabethkingia bacteria, which was named after the microbiologist who identified it in the 1950s, Braden said. Of those cases, 17 people have died, but investigators don’t know if they died from the infection, an underlying medical condition or both.

The Michigan resident also died.

The Wisconsin outbreak — the largest one that health officials know about — and the mystery surrounding it have Minnesota officials watching to see if it pops up in their state.

Although the bacteria is common in the environment, including in water and soil, it rarely causes infections, and the risk is very low in young or healthy people. It can, however, be a problem for those with compromised immune systems, Braden said. The latest outbreak involves patients who are either very elderly or have serious underlying medical conditions, such as cancer, liver disease or kidney disease, he said.

Most previous outbreaks have occurred in a specific place, such as a hospital, Braden said. Not only is the Wisconsin outbreak larger than any that he’s known, it also isn’t associated with any particular location or facility, Braden said. “These patients are in different places around southeastern Wisconsin,” he said. Many aren’t even mobile because they are homebound or in a nursing home. “They’re on different water systems, some have their own private wells. There’s no commonality for some particular environmental exposure that people had.”

And yet, the investigation led by Wisconsin health officials with help from the CDC is steering them toward the idea that there’s a common source, Braden said. Finding it has them stumped so far.

The culprit could be a product that was manufactured in a plant contaminated by the bacteria, Braden said.

“That’s the theory, but we haven’t been able to find what it might be or how something could have been contaminated,” he said. “We’re looking hard at all sorts of different products that these people may have been exposed to — personal care products or medical products. Just think of yourself and how many things you put on your skin or you ingest in some way each day. We’re trying to figure out if there’s anything in common.

“It’s painstaking field work,” with health officials asking 300 to 500 questions of each person who was infected or the families of those who died, Braden said. “But the longer we go without a lead to a common source, then you start to wonder whether there is one,” he said. “But we’re not there yet.”

Symptoms that can result from being exposed to the bacteria include fever, shortness of breath, chills or cellulitis. Health officials say the bacteria tend to be resistant to many common antibiotics, making early detection critical.

Health officials in neighboring states like Minnesota have sent alerts to medical facilities and labs to review previous records dating back to Jan. 1, 2014, for the Elizabethkingia bacteria and watch for new cases, Braden said.

Doug Schultz, spokesman for the Minnesota Department of Health, said two people last year suffered infections from Elizabethkingia bacteria but neither was an exact match to the one in Wisconsin or Michigan. Each of those patients had underlying medical conditions, Schultz said, and one died.

“The risk for the average person is very low,” Schultz said. “And because we haven’t seen it in Minnesota, the risk would be very, very low.”

Still, Schultz cautioned that people ought to practice good infection control and good hygiene in settings where people might be more vulnerable to illnesses.