The largest federal study to date into the causes of autism was announced Friday — a multi-state investigation that will involve 2,700 young children.

The U.S. Centers for Disease Control and Prevention and five other research centers will study the youngsters over five years. The research is designed to ferret out any genetic and environmental factors that may contribute to autism.

"The CDC hasn't funded a study like this (before)," said Diana Schendel, the CDC lead health scientist who is overseeing the Georgia research.

But some parents of autistic children say the CDC — which promotes childhood vaccinations — is not interested in fully exploring vaccinations as a potential cause.

"We don't want the CDC to do anything. We don't trust them," said Wendy Fournier, president of the National Autism Association.

Autism is a complex disorder usually not diagnosed in children until after age 3. Symptoms can include repetitive behaviors such as head-banging, avoiding physical or eye contact with others, and communicating with gestures rather than words.

In 2000, Congress directed federal health officials to increase research into autism. The law prompted a series of CDC studies, including prevalence research released in May that found 300,000 U.S. children have been diagnosed with autism.

The new study will recruit 900 children diagnosed with autism, 900 with undefined or other developmental problems, and 900 randomly selected youngsters.

Those studied will be ages 2 to 5, in part because health records and memories will be more complete, Schendel said.

That decision will limit the study's ability to assess the past impact of vaccinations that contain the mercury-based preservative thimerosal, she acknowledged. Since 2001, thimerosal has been removed from shots recommended for young children.

Fournier's group suspects that ingredient is a leading cause of the disorder, although past research suggests it is not.

Researchers will examine the medical records of the children and their parents, and will take cheek swabs and blood and hair samples, Schendel said.

The CDC awarded the other participating institutions $5.9 million for the study. They are the Kaiser Foundation Research Institute in California, the Colorado Department of Public Health and Environment, Johns Hopkins University in Maryland, the University of North Carolina at Chapel Hill and the University of Pennsylvania.

Until Friday's announcement, the largest federal study to focus specifically on autism's causes was research sponsored by the National Institute of Environmental Health Sciences, looking at 1,000 California children ages 2 to 5. That study is still in progress.