The belief -- only that, for the moment -- in Fort Chipewyan is that something from the oil sands is contaminating the Athabasca and ravaging the health of the people who live downstream.

Nearly four years after the hamlet's only doctor first voiced concern about the cluster of cancer cases, the provincial and federal governments have launched a joint investigation into the illness that seems to be sweeping Fort Chipewyan.

The Athabasca and Mikisew people are waiting for answers from those officials, but in the meantime, they have their own explanations.

"It is speculation to say it's the water. But for me, it's common sense," said Lorraine Mercredi, who bought a water-filtration system after her aunt and a cousin, still in his early thirties, died from cancers of the digestive tract. Other Fort Chipewyan residents, too afraid to drink from their taps at all, are paying to have bottled water flown in.

When Ivy Simpson was diagnosed as having cervical cancer, her doctors did not tell her what had made her ill. But the Fort Chipewyan resident, who now lives about 250 kilometres to the south in Fort McMurray, has no doubt about what caused her cancer.

"It had to have been something from the water, air or land," said the 27-year-old, who was just 17 when she contracted cervical cancer, a disease usually found in much older women.

Her extended family in Fort Chipewyan has been hit hard by cancer. Her mother, Mary Simpson, said a cousin, Warren, got testicular cancer. An aunt died of uterine cancer in the late 1980s, and Ivy Simpson's 41-year-old sister has terminal cervical cancer.

Like many in Fort Chipewyan, the Simpsons began to suspect their surroundings were making them sick after the town's fly-in doctor, John O'Connor, began to push for an official inquiry into what he saw as an astonishingly high number of cancer cases.

A few months after arriving in 2001, Dr. O'Connor noticed a set of disturbing symptoms in a patient: yellowed eyes, fatigue and abdominal discomfort. It was disturbing not only because it pointed to cholangiocarcinoma, a rare and deadly cancer of the bile duct. The symptoms were all too familiar for Dr. O'Connor, whose father died of the cancer 13 years ago in Ireland.

"I know a lot about it, but I never expected to see it again," he said. "Without treatment, you're dead in about a month. My dad lasted six weeks." Dr. O'Connor said at least three residents of Fort Chipewyan, and likely another two, have died of the disease within the past five years. Statistically speaking, there should be only one case for every 100,000 people, and none at all for a community the size of Fort Chipewyan, he said.

There are similar patterns with other serious diseases. Since 2001, he has diagnosed five cases of leukemia and four cases of lymphoma, a cancer that originates in the lymphatic system. In the past year, Dr. O'Connor has treated at least six patients with Graves' disease, an immune-system ailment, and has seen entire families stricken with lupus, another serious autoimmune disease. So far this year, six people have died of colon cancer, the youngest just 33 years old, the doctor said.

He visits a number of northern communities in his weekly rounds, and no other has been hit by the kind of cancer cluster seen in Fort Chipewyan. Those other communities do not draw their drinking water from the Athabasca River or the lake, however. Faced with those seemingly unique numbers, Dr. O'Connor said he cannot help but believe the cancers in Fort Chipewyan are linked to industrial development elsewhere on the water system. The pulp-and-paper industry is a possibility, he thinks. So is Uranium City, Sask., on the northeast shore of Lake Athabasca, where mining activity ceased years ago, but contamination lingers. Then there are the oil sands, which have been producing bitumen using river water for decades and are heading into a massive expansion.

Much of the province's future prosperity hinges on that growth, a reality Dr. O'Connor believes at least partly explains why it took more than three years for the provincial and federal governments to launch a formal investigation.

Now that investigation is under way. A team of doctors from Edmonton employed by the federal government was in Fort Chipewyan last week laying the groundwork for a rigorous statistical study aimed at determining whether the hamlet has an unusual incidence of cancer, and if it does, what is causing it. Health Canada, Alberta's Health and Wellness department and the Alberta Cancer Board are all taking part in the investigation, the only one of its kind in the province.

"We take these concerns very seriously," said Salim Samanani of Health Canada's First Nations and Inuit Health branch.

Alberta Health and Wellness Minister Iris Evans said government officials "are all over" the alarming medical concerns coming out of the northern Albertan community. She said her ministry is working with the provincial cancer board on an "intensive analysis" of the problem.

Investigators say it will take up to three months to assemble statistics, in part because the team will have to acquire all the data, including confirmations of Dr. O'Connor's diagnoses. One of the first tasks will be to determine whether the massing anecdotal evidence of a rising number of cancer cases is borne out by statistics; whether there is, in fact, a cancer cluster. Another major difficulty is that the kinds of cancers cropping up in Fort Chipewyan have multiple causes. Ms. Simpson believes environmental contaminants made her ill, although the main risk factor for cervical cancer is the human papilloma virus. Another possible contributor is a diet poor in fresh fruits and vegetables, a near certainty in Fort Chipewyan, where a head of cauliflower costs $7.

Dr. Samanani of First Nations and Inuit Health said he is sure of at least one thing: the drinking water, which is filtered through modern treatment facilities, is safe.

"I felt quite comfortable coming in and drinking the water," Dr. Samanani said, later draining a tall glass of tap water in the staff kitchen of Fort Chipewyan's nursing station.

However, it is equally certain that residue from the oil sands is being pumped into the Athabasca River. The oil-sands industry uses water to separate bitumen, which can be turned into crude oil, from sand. Some operators then treat portions of that water and return it to the river, where it eventually flows into Lake Athabasca -- and the water supply of Fort Chipewyan.

Suncor Energy Inc., the oldest oil-sands operator, does discharge "process water" into the river. However, the company emphasizes that it has dramatically reduced in recent years the overall volume of water it returns to the Athabasca as it has improved its recycling efforts, and that the hydrocarbons and other chemicals that are in the water are far below Alberta's limits.

With a report from Katherine Harding