New research has found that mercury levels in young people with autism are no higher than those without the illness.
Experts recently rejected findings from the Institute of Medicine in the US, which showed that the mercury-containing vaccine thimerosal could not be blamed for autism.
However, further research has now found that there is no difference between mercury levels in children with autism and the kids who did not have the condition.
The findings have been published online by the journal Environmental Health Perspectives.
Ongoing research is being carried out by a team at the UC Davis MIND Institute, who are looking at genetic and environmental risks for autism.
In a statement on the work, lead scientist Irva Hertz-Oicciotto said: "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 added that it was time to abandon the idea that a single smoking gun will emerge to explain why so many children are developing autism.
In Great Britain alone, there are over half a million people with autism, according to the National Autistic Society.