Tissue samples taken last September from a Chilmark skunk and a Katama rat tested positive for tularemia, the rare disease that infected 15 people on the Island last year, killing one man who did not seek medical attention in time.

Those bits of evidence, scientists admitted last night at a public forum in Oak Bluffs, hardly bring public health experts closer to knowing why an outbreak of tularemia hit the Island last year. Even after months of research and weeks of field studies here in which 40 mammals were trapped and dozens of air, soil and grass samples were collected, the experts don't yet have a solid theory.

"What can we say about why it happened?" asked Dr. Katherine Feldman, an epidemic intelligence officer from the federal Centers for Disease Control (CDC). "Not a lot."

But if the experts are still trying to solve the mystery of why, at least they felt sure about what put most of the victims in the path of the disease. It's lawn mowing, they said.

That conclusion confirms what many doctors and epidemiologists suspected from the start, but yesterday, they had data to back up their suspicions. Of the 15 confirmed cases of tularemia, 12 people had been mowing a lawn or cutting brush two weeks prior to getting sick, Dr. Feldman said.

The problem with mowing is that it puts people at risk for inhaling airborne particles contaminated with the bacteria that causes tularemia. Still, the fact that so many of the cases could have contracted tularemia in this manner is especially puzzling to the experts.

That's because the far more common disease vector is a bite from a dog tick, which passes the bacteria from infected animals, such as rabbits or rodents, to humans.

On the Island, though, the opposite held true. Of the 15 cases, 11 were the pneumonic form, definitely caused by breathing in the bacteria.

What became clear last night is that the Vineyard now holds an unusual place in medical history. "Martha's Vineyard has a dubious honor," said Dr. Feldman, of having experienced the country's only two outbreaks of pneumonic tularemia. In 1978, seven people staying in the same cabin in Chilmark came down with tularemia. One theory was that all seven breathed in contaminated particles when a dog came inside and shook itself. That same year, eight more cases of tularemia were confirmed, seven of them pneumonic.

The connection to 1978 intrigues CDC investigators who are still analyzing and comparing weather data between last summer and 1978. It's possible that similarities in temperature and rainfall would have been ideal for keeping the bacteria alive.

Geographic results of the study pointed to a cluster of exposures to the bacteria in the Katama area of Edgartown and the Squibnocket region in Chilmark. When the CDC team of investigators came to the Island twice last summer, these are the areas where they spent most of their time, collecting samples to bring back to their lab in Fort Collins, Colo.

A slide presentation last night showed photographs of these investigators, dressed in full white protective suits and headgear as they mowed lawns and held gadgets that were collecting air samples. It looked like a scene from the X-Files. "We tried to be discreet so as not to alarm people," Dr. Feldman said.

But in spite of all their efforts, Dr. Feldman said, many of the lab results were disappointing. As potent as the tularemia bacteria is, it is not easy to culture in the lab. The CDC team had collected tissue samples from one Eastern cottontail rabbit, one house mouse, two meadow voles, 23 white-footed mice, 11 Norway rats and two striped skunks. Only one rat and one skunk tested positive, but Dr. Feldman pointed out that the CDC is still planning to conduct a DNA test on the tissue samples called a polymerase chain reaction (PCR) that could turn up different and more reliable results. She could not say when those tests would be complete and the results made public.

One problem with trapping animals to get information on tularemia, Dr. Feldman said, is that most mammals with the disease are sick or dying, and not likely to be venturing into a trap.

While Dr. Feldman avoided committing to any theories about what caused the tularemia outbreak, another panelist, Dr. Michael McGuill, a veterinarian at the state Department of Public Health (DPH), took a crack at it. "Some animals can excrete the (bacteria) in their urine and contaminate the soil," he said.

But while scientists know that voles pass the bacteria though their urine, they are not certain about other animals.

Indeed, hard evidence was in short supply last night, so the panel focused instead on preventive measures while also fielding questions from the nearly 30 people who had come out for the session. Susan Soliva, an epidemiologist from the DPH, distributed a new public health advisory on tularemia aimed at residents of the Vineyard. A CDC survey of 100 Islanders found that 30 per cent had mowed their lawn in the previous two weeks.

Far more expansive that last year's advisory, this one urged people preparing to mow a lawn or cut brush to check an area first and make sure it is clear of animal carcasses. It advised gently placing any carcasses in a double bag of plastic before putting it in the trash. The advisory recommended wearing respirators or dust masks if there is a great risk of inhaling contaminated air particles. These products should be stamped with an approval by the National Institute of Occupational Safety and Health (NIOSH). But the advisory also states that the effectiveness of such protection against airborne tularemia has not been proven.

People were curious to know whether they could get tularemia from their pets and whether the bacteria could survive in the winter. Dr. McGuill said that humans can contract tularemia from their cats. If a cat shows signs of fever, loss of appetite or listlessness, owners should call a veterinarian.

Cases of tularemia in the winter are rare, Dr. Feldman said, but she also pointed out that the last case of tularemia here was contracted in late October.

It was Dr. Dennis Hoak, a panelist and an infectious disease specialist who spotted the first tularemia case in June, who raised the issue of bio-terrorism. The tularemia bacteria is known to be a weapon used in biological warfare and by terrorists, but Dr. Feldman assured people that the outbreak here had nothing to do with terrorism. Still, the CDC had not ruled that out last summer when they sent investigators here who specialize in this area.

Vineyarders, however, are not alone in worrying about tularemia. Dr. Feldman had done her homework on the international scale, finding out about an outbreak in Kosovo where infected rats got into a public water supply. Elsewhere in Europe, she said, farmers came down with the pneumonic version when their hay became contaminated.