State health investigators are intensifying efforts to pinpoint the source of the Hopkins outbreak of Legionnaires’ disease even as three new cases were confirmed Friday.

The outbreak has now sickened 23 people and led to one death.

After weeks of testing and disease tracking, epidemiologists at the Minnesota Department of Health still have not found ground zero of the outbreak, but they are looking at a likely culprit.

“We really think it is consistent with a cooling tower exposure rather than any other water source,” said Richard Danila, assistant state epidemiologist, who is leading the investigation.

That would resemble many other outbreaks across the country. Contaminated cooling towers for large air conditioners release an aerosolized vapor that can spread the bacteria that cause Legionnaires’ over a wide area. Not everyone who breathes the vapor gets sick, but people with compromised immune systems, chronic illnesses or a history of tobacco use are more prone to get infected.

State health officials said ­Friday they are redoubling their efforts to identify all cooling towers in the area around downtown Hopkins, a task that has proved difficult because there is no central government registration or regulation of air-conditioning systems.

When the first cluster of Hopkins-related cases was recorded in early September, health officials immediately moved in and identified several cooling towers in the area. They took water samples, then took steps to disinfect the towers. If not properly maintained, such cooling towers can foster an environment for the naturally occurring legionella bacteria to grow.

Health officials hoped those steps would stem the outbreak and, until Friday, all the reported cases could be traced back to exposure that occurred before the suspect towers were disinfected.

But two of the three new cases probably began after those remediation steps were taken, Danila said.

Also, investigators this week found two additional cooling towers in the area they had previously missed. They are now being tested and disinfected.

“If we continue to see more cases, we’ll have more evidence that it wasn’t the initial cooling towers that we identified, but the more recent ones,” Danila said.

Using Google Earth

Without a central registry, investigators have turned to satellite imagery from Google Earth to spot additional cooling towers. But even then, the search has been difficult because cooling towers differ in size and placement, and sometimes resemble other heating, venting and air-conditioning systems.

“We’ve been working with individual building owners and their contractors to make sure that they are aware of [cleaning] guidelines and follow them for remediation,” Danila said.

Altogether, cooling towers at seven Hopkins businesses are being examined as possible sources.

Danila said testing of water samples from the newly identified cooling towers could take up to two weeks to complete. “Testing takes a long time and interpretation of results is difficult,” he said.

Minnesota has also sent water samples to the New York state health laboratory for confirmatory tests. That lab has recent experience from a 2015 Legionnaires’ disease outbreak that sickened more than 130 people in the Bronx borough of New York City. Eventually, the cause of that outbreak was linked to cooling towers.

No common exposure point

Danila said interviews with those sickened during the Hopkins outbreak have failed to identify any one confined exposure point, such as a restaurant or grocery store. Produce misters at one grocery store were tested, but Danila said the store was not a source because many of those sickened never visited it.

The outbreak comes at a time when Legionnaires’ cases are on the rise nationally and in Minnesota.

“Even without the Hopkins outbreak we are on for a record-breaking year,” Danila said. To date, 90 cases have been identified in the state, compared with no more than 60 cases in each of the past four years.

Once identified, Legionnaires’ is often successfully treated with antibiotics. Symptoms can include fever, chills, headaches, muscle aches, fatigue, shortness of breath, cough, mental confusion, diarrhea and vomiting. Severe cases result in pneumonia and sometimes death.