The preliminary report into the latest outbreak of bird flu in Suffolk concludes that wild birds "may have been" the main source of infection.
Thousands of birds were culled as a result of the Suffolk outbreak
But no evidence has yet been found to support this theory.
The report also found that poor biosecurity was practised by some of the staff on the farm where the outbreak occurred on 11 November.
This is thought to be the probable reason for the disease spreading to another farm.
The virulent H5N1 strain of the virus, a variant capable of being transmitted to humans, was first discovered at Redgrave Park Farm near Diss, where all 6,500 birds were slaughtered.
The disease was later also confirmed to have reached the nearby Hill Meadow Farm at Knettishall - 9,000 turkeys were culled there.
The report concludes that the disease has a very close genetic match with an outbreak of H5N1 in the Czech Republic in the summer.
It says that there are two main theories about how the disease was introduced to the UK - either via contamination from people from a country with an undisclosed infection in their domestic flocks, or infected wild birds.
There is currently no evidence to support either theory.
"As there are no epidemiological links with domestic poultry in central Europe, the molecular genetic results suggest that wild birds may have introduced the virus into Suffolk from Europe," the report said.
The Department for Environment, Food and Rural Affairs (Defra) says the disease was discovered in an area where wild birds were relatively common and was notably near to an ornamental lake which supports some 1,000 waterfowl.
The affected poultry were free-range - meaning they had access to the outdoors and may have been at greater risk of catching the disease.
H5N1 infection has not been detected in wild birds nor have any incidents of high mortality been observed in the area, according to the report.
But it added that wild birds, most likely migratory species from central Europe, cannot be ruled out as the source of infection.
The report also said poor biosecurity was employed by stockmen who worked at Redgrave Park Farm and on other farms in the area.
Simple measures to prevent the transmission of infection between premises were not followed, it said.
Such measures include changing clothing between premises, disinfection of Wellington boots, the disinfection between premises of buckets for the distribution of feed, and the carriage and handling of dead birds.
Gressingham Foods' subsidiary Redgrave Poultry, which runs both Redgrave Park Farm and Hill Meadow said it had identified several changes that were needed when it purchased the farms at the start of the year.
It said: "Due to our commitments to our customers and the lack of available organic land, it was simply not practical to make all of the planned changes for this season.
"We believe in the highest standards of organic and free range farming for these production systems, so we have looked for lessons from the recent outbreak and have identified a number of improvements that we are implementing."
Dr Mark Avery, conservation director for the RSPB, said imported poultry could have been the cause of the Suffolk outbreak.
"A migrating bird could have carried the disease here without showing symptoms but imported poultry could have done exactly the same.
"Defra and the poultry industry should be doing more to protect wild birds from coming into contact with infected farmed birds."
Suffolk previously had an H5N1 outbreak at a turkey farm in February, but the report found there was no connection that incident.