CHICAGO (AP) — The most comprehensive study of potential World Trade Center-related cancers raises more questions than it answers and won’t end a debate over whether the attacks were really a cause.

The study suggests possible links with prostate, thyroid and a type of blood cancer among rescue and recovery workers exposed to toxic debris from the terrorist attacks. But there were few total cancers and even the study leaders say the results “should be interpreted with caution.”

The study involved nearly 56,000 people enrolled in a registry set up to monitor health effects from those exposed to the aftermath of the trade center attacks. Most participants volunteered for enrollment, which could skew the results if people who already had symptoms were more likely to enroll than healthier people.

Cancers diagnosed through 2008 were included in the study, but that’s just seven years after the 2001 attacks, and cancer often takes longer to develop. People diagnosed with cancer before the attacks were excluded from the study.

Cancer rates were compared with those in the general New York state population. But the researchers had no data on whether people in the study had risk factors for getting cancer, including a strong family history, or if they had existing cancer that wasn’t detected until after the disaster. Participants are being monitored for health issues and may have gotten more cancer screening than other people, which also could skew the results.

The increased risks were seen only in rescue and recovery workers, who likely had more direct, sustained contact with potential cancer-causing substances in the dust, smoke and debris from the attacks. But cancers weren’t more common in workers who had the most exposure – a finding that would seem to contradict the theory that contact was the cause.

The study comes just a few months after the federal government added dozens of types of cancer to a list of illnesses related to the trade center attacks that will be covered by a program to pay for health coverage.

The study results “won’t settle the question because it’s still too early,” said Dr. Thomas Farley, New York City’s health commissioner. “People are very, very interested in this topic and we thought it was important to get the data out that we have even though it is early.”

Marijo Russell O’Grady, dean of students at Pace University’s New York City campus, was at her office near the trade center during the attacks. She also lives nearby, and said she worries about how exposure to choking dust, ash and an “overwhelming burnt plastic smell” might affect her family, including her then 1 1/2 year-old son. They are all enrolled in the health registry.

Cancer is her greatest concern and it’s “always present in the back of my mind,” she said.

Researchers from the city’s health department led the study, which was partly paid for by the National Institute for Occupational Safety and Health. NIOSH spokesman Fred Blosser said the agency welcomes the results and that longer follow-up is needed to examine risks for cancers with that take a long time to develop.

The study appears in Wednesday’s Journal of the American Medical Association.

Earlier research from the same registry linked the attacks with respiratory problems including asthma and symptoms of post-traumatic stress.

The new study involved a broader array of people, including firefighters and other emergency workers, along with residents and employees of workplaces near ground zero, Farley said.

In the new study, possible links were mainly seen with cancers diagnosed in 2007 and 2008 in rescue and recovery workers. These included 67 cases of prostate cancer, 13 thyroid cancer cases, and seven cases of multiple myeloma – all at rates higher than in the New York state population.

Donald Berry, a biostatistics professor at the University of Texas MD Anderson Cancer Center in Houston, said the study has too many limitations to draw any definitive conclusions.

“There’s no evidence that 9/11 caused any of these cancers,” Berry said.

He pointed out that no increased risks were found for lung cancer – a cancer that might seem plausible after breathing lots of toxic dust and smoke.