RALEIGH, N.C. - A long-awaited study by the U.S. Centers for Disease Control and Prevention shows a link between tainted tap water at a U.S. Marine Corps base in North Carolina and increased risk of serious birth defects and childhood cancers.

The study released late Thursday by the CDC's Agency for Toxic Substances & Disease Registry is based on a small sample size and cannot prove exposure to the chemicals caused individual illnesses. It surveyed the parents of 12,598 children born at Camp Lejeune between 1968 and 1985, the year most contaminated drinking water wells were closed.

The study looked back in time and was designed to see if there was a link between exposure to certain chemicals and certain health problems that developed later. This type of study is often used to investigate disease outbreaks, when health officials are trying to identify possible reasons for the illnesses.

The study concludes that babies born to mothers who drank the tap water while pregnant were four times more likely than women in similar circumstances who did not consume the water to have such serious birth defects as spina bifida. Babies whose mothers were exposed also had a slightly elevated risk of such childhood cancers as leukemia, according to the results.

The study relied on models and was not able to measure how much tainted water those surveyed consumed, and therefore could not gauge how much of the chemical they may have been exposed to. The study also did not look at the health effects on adults that drank the water. More than 80 men with Lejeune ties have been diagnosed with an extremely rare form of breast cancer.

Epidemiologist Richard W. Clapp, who serves on a federal board that has reviewed the Lejeune contamination, said the links found through the study are relatively weak due to the relatively small sample size. Of the families surveyed, 106 cases of birth defects and childhood hematopoietic cancers were reported. Of those, medical records to confirm the illnesses could be obtained in only 52 cases.

Still, Clapp said the findings are important because they show the first conclusive links between the base's tainted water and negative health effects in the children of Marines based there.

"The fact that there was anything found is pretty important," said Clapp, professor emeritus at Boston University's School of Public Health. "This is an insensitive tool that we use here, these epidemiological studies. So the fact that they found anything is sort of remarkable."

The study contradicts the longstanding position of the military, which for decades has issued public statements downplaying the health risks to Marines and their families.

A brief statement issued by Lejeune spokeswoman Capt. Maureen Krebs said the Marine Corps has supported scientific and public health organizations studying the health impacts of the contamination.

"These results provide additional information in support of ongoing efforts to provide comprehensive science-based answers to the health questions that have been raised," the statement said. "The Marine Corps continues to support these initiatives and we are working diligently to identify and notify individuals who, in the past, may have been exposed to the chemicals in drinking water."

Krebs said Friday she could provide no comment about the new report beyond the written statement.

Records reviewed by The Associated Press show military authorities continued to rely on the wells for years after testing suggested the water was contaminated. The most highly contaminated wells were closed in 1984 and 1985, after a round of more extensive testing found dangerous concentrations of toxins associated with degreasing solvents and gasoline.

A prior CDC study cited a February 1985 level for trichloroethylene of 18,900 parts per billion in one Lejeune drinking water well - nearly 4,000 times today's maximum allowed health limit of 5 ppb. Testing also found high levels of benzene, a fuel additive.

The ground water contamination was traced to two primary sources - a leaky on-base fuel depot and a nearby dry cleaner. In prior public statements, Marine officials have emphasized the contamination that came from outside the base. But the newly released study found the greatest negative health impacts to be associated with benzene, which came from the on-base Hadnot Point tank farm built during World War II.

Last year, President Barack Obama signed the Camp Lejeune Veterans and Family Act to provide medical care and screening for Marines and their families, but not civilians, exposed between 1957 and 1987. The law covers 15 diseases or conditions, including female infertility, miscarriage, leukemia and multiple myeloma, as well as bladder, breast, esophageal, kidney and lung cancers.

The law was passed after years of advocacy by former Marines who blamed the contamination for negative health impacts, efforts that were often met with strong resistance from the Marine Corps.

Jerry Ensminger, a former Marine drill instructor, lost his 9-year-old daughter Janey to leukemia in 1985. He said the study results are a vindicadation of what he's been saying for nearly 20 years, but it won't bring his daughter back.

"Nothing ever gives you comfort when you lose a child," Ensminger said Friday. "I think that's the worst thing that can happen to a human being ... to watch them go through the hell they go though. That's something that never leaves you."

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Associated Press national writer Allen G. Breed contributed to this report.