Six months after the federal government added cancer to the list of sicknesses covered by the $4.3 billion World Trade Center fund, a New York City health department study has found no clear link between cancer and the dust, debris, and fumes released by the burning wreckage of the towers.

The study was by far the largest to date. It examined 55,700 people, including rescue and recovery workers at the World Trade Center site, on barges, or at the Staten Island landfill where debris was taken, in the nine months after Sept. 11, 2001, as well as residents of lower Manhattan, students, workers, and passersby exposed on the day of the attacks.

Overall, there was no increase in the cancer rate of those studied compared with the rate of the general population, researchers concluded after looking at 23 cancers from 2003 to 2008. The prevalence of three cancers was significantly higher - multiple myeloma, prostate, and thyroid - but only in rescue and recovery workers, and not in the rest of the exposed population.

And the researchers noted that those were very common cancers and that the number of people who received diagnoses of them was small. In one of many counterintuitive findings, the incidence of cancer was not higher among those who were more intensely exposed to the toxic substances than among those who were less exposed.

Given the lack of evidence of a link between 9/11 debris and cancer, some epidemiologists had questioned the decision by the government in June to add 50 types of cancer to the list of illnesses covered by a law signed by President Obama in early 2011. That decision to add cancer to the list meant that people with other sicknesses more strongly linked to ground zero were likely to receive less money.

Thomas Farley, the health commissioner in New York City, said in an interview that it was too soon to take the study as a repudiation of the government's decision.

"Cancers take 20 years to develop," Farley said, "and we might see something different 20 years down the line." But echoing John Howard, head of the National Institute for Occupational Safety and Health, who made the final decision on covering cancer, the commissioner added: "You don't want to wait 20 to 30 years to get a definitive answer to which people may be suffering today."

Until now, the only systematic examination of cancer incidences and Sept. 11 was a study by the New York Fire Department released last year. It found a 19 percent higher incidence of all types of cancer for exposed firefighters compared with those not exposed.