The Minnesota Department of Health is investigating an "extremely rare" case of inhaled anthrax, but officials said Tuesday there is no threat to the public.

The patient, who was not identified, was hospitalized in Minnesota after traveling through Montana, Wyoming and the Dakotas, where anthrax spores occur in the environment.

The infectious agent probably came from natural sources, and there is no evidence of a link to terrorism, according to a Health Department statement released Tuesday. Nevertheless, the FBI joined in the investigation because anthrax can be used as a bioterrorism agent.

The case most likely occurred when the individual was exposed to soil or animal remains infected with anthrax, a bacterial spore that can cause potentially fatal illness.

"We're not giving details about the case simply because it is not a risk to the public, and we're concerned about patient privacy," said Dr. Ruth Lynfield, the Minnesota state epidemiologist. She noted that anthrax is not spread from person to person.

She said the patient is being treated, but declined to specify his or her medical condition.

The patient, who is not from Minnesota, was hospitalized somewhere in the state after developing fever and pneumonia. When the hospital conducted routine specimen tests, the lab discovered an unusual growth and sent it to the Health Department for further testing.

Lynfield said her office confirmed it as anthrax, and contacted the FBI as "standard protocol."

Government officials have treated anthrax exposure as a possible act of terrorism since the deadly spate of anthrax-tainted letters that followed the Sept. 11 terrorist attacks in 2001.

That year, 22 people developed anthrax infections and five died after handling or receiving letters containing anthrax spores. In spite of intensive investigation, no one was charged in that case.

In the latest case, though, investigators found "no evidence suggesting it was a criminal or terrorist act," the Health Department said, adding that the FBI is no longer actively investigating the incident.

Lynfield said such cases are "extremely rare," although in the past anthrax was known as "woolsorter's disease" because people would breathe in spores from infected animals. Anthrax can infect cattle, sheep and other animals, and can live in soil for years.

Inhaled anthrax is far more dangerous than a more common type of anthrax infection, which infects the skin.

The illness can start with flu-like symptoms, such as cough, fever and muscle aches, and last several days before seeming to disappear. When it returns, it can cause lung problems, breathing difficulty and shock. Without treatment, it is fatal in up to 90 percent of cases, according to the Health Department.

The department is continuing to investigate.

Maura Lerner • 612-673-7384