Children with autism have mercury levels similar to those of other kids, suggesting the mysterious disorder is caused by a range of factors rather than "a single smoking gun," researchers said on Monday.

The researchers at the University of California, Davis, initially found that children aged 2 to 5 with autism had mercury levels lower than other children because the autistic kids ate less fish, the biggest source of mercury that shows up in the blood.

But when the data were adjusted for lower fish consumption, blood-mercury concentrations among the autistic children were roughly similar to those developing typically. The children with autism had mercury levels in line with national norms.

The findings, published online in the journal Environmental Health Perspectives, come at a time when advocates including parents argue that mercury found in fish, dental fillings, vaccines and industrial emissions are responsible for autism.

The debate became more vehement this month after the U.S. Centers for Disease Control and Prevention said autism was more common than previously thought, affecting one in 91 children, including about one in 58 boys.

"It's time to abandon the idea that a single smoking gun will emerge to explain why so many children are developing autism," said Irva Hertz-Picciotto, who led the study.

"Just as autism is complex, with great variation in severity and presentation, it is highly likely that its causes will be found to be equally complex," she said in a statement.

Autism refers to a spectrum of diseases, from severe and profound inability to communicate and mental retardation to relatively mild symptoms. The research area is due for a large infusion of money from President Barack Obama's $5 billion plan to boost U.S. medical and scientific research.

Activists noted that the University of California study did not seek to find out whether mercury might cause autism because the children's blood levels were measured after autism had been diagnosed.

"The results of this study are limited in terms of ruling in or out a link between mercury exposure and autism causation or severity," said Sallie Bernard, executive director of the advocacy group SafeMinds.

Vaccines with mercury-containing preservative called thimerosol have also been blamed by some parents as a potential cause of autism, although many studies and several reports from the Institute of Medicine have found no link.

University of California Davis researchers looked at 452 children, including 249 with autism, 60 who had other developmental problems including Down's syndrome and 143 children without disorders.

They also examined a variety of mercury sources including fish, nasal sprays, earwax removal products, vaccinations and dental fillings made from a mercury-based amalgam.

Autism researchers are looking at a broad range of environmental factors including household products, medical treatments, diet, food supplements and infections. Other recent studies have found strong evidence of several genetic causes for autism.

"The evidence to date suggests that, without taking account of both genetic susceptibility and environmental factors, the story will remain incomplete," Hertz-Picciotto said.

"Few studies, however, are taking this kind of multifaceted approach."