I think genetics may play a stronger role than the environment. Since it's something from birth, and usually not something that's developed later in life, there may be a biological or genetic component. I do wish that parents and loved ones of autistic children can one day receive answers.
I don't think the true etiology is known at the present time. There has been some alleged link to childhood immunizations but I don't think that has been proved. Also, I don't think we can point to a specific genetic situation to say for sure the genetics are the root cause.
With the number of young children being diagnosed as Autistic and being evaluated as "on the spectrum" growing all the time, it is vital that we learn more about this complex disorder. Unfortunately, no one can answer your question at this time. There are thousands of researchers out there with theories, but it is going to take more time before we can say "these are the causes" and "these are the definitive solutions."
I think there are genetic and environmental factors that make some children more susceptible to being autistic but that there are likely multiple factors that ultimately determine whether a not a person shows symptoms of autism and how severe they symptoms are. I think the research currently being done hasn't come to a conclusive finding on what causes autism but I would expect that within the next 5-10 years, there will be a much greater understanding of its origins and possibly things we can do to prevent or minimize the effects.
I believe genetics does play a role in the people that have been diagnosed with autism, but it hasn't yet been proved, and therefore is an invalid statement. It's still hard to say how people obtain diseases, and whether genetics plays a role in the capture of disease is unknown.