There is a "strong probability" the foot-and-mouth outbreak began at a research site, inspectors have said.
The disease has been identified at two farms in Surrey
But either the private company Merial, or the state-run Institute for Animal Health, both based at the Pirbright site, could be the source, they said.
The Health and Safety Executive found there was a "negligible" risk it had been spread by the wind or flooding.
But its report said the disease could have been the result of human movement or "accidental or deliberate transfer".
The National Farmers' Union (NFU) has said there is "incredulity and shock" that a research facility that works to protect against disease could have been the source of the outbreak.
HOW FOOT-AND-MOUTH SPREADS
Direct contact, from animal to animal
Fluid from an infected animal's blister; saliva, milk or dung also pass on the disease
Animals eating infected feed
Virus can be spread by people, vehicles or roads, if not disinfected
Airborne spread of disease also possible
Animals can begin spreading virus before visible signs of disease emerge
The first cases were found at Woolford Farm near Guildford in Surrey on Friday, and a second outbreak was confirmed at a second farm on Monday. Both farms are within miles of the Pirbright site.
The HSE's interim report confirmed the strain found in the first outbreak was the same as that being worked on at the nearby Pirbright site and there was a "strong probability" it originated there.
But it did not specify which of the two facilities on the site was to blame.
Vaccine manufacturer Merial had been involved in "large scale production" of the strain - about 10,000 litres - while the Institute for Animal Health (IAH) had been carrying out "small scale" experiments, it said.
Microbiology expert Hugh Pennington said it was possible there had been "some sort of surface leak at Pirbright".
"Then perhaps feet, motor car tyres or something like that could have transported it down a bit further south into Surrey," he told BBC News.
Airborne risk 'negligible'
In a statement, Merial Animal Health said: "Merial is assessing the information contained. We will communicate further as soon as possible."
Head of the IAH, Professor Martin Shirley, said the Institute was concerned about the "lack of unambiguous evidence" at this stage of the investigation and it would continue to review its biosecurity measures.
He added: "We continue to be concerned about the effect on the farming community in the UK and you can be assured that our staff are working as hard as possible to provide evidence for these inquiries."
The HSE report said there was no evidence that working practices, or any spills or leaks from equipment were to blame and the risk of water or airborne transmission was considered "negligible".
But there were "various potential routes for accidental or deliberate transfer of material from the site" and various lines of inquiry were being pursued.
"Release by human movement must also be considered a real possibility," the report said.
Prime Minister Gordon Brown said he was "hopeful" a second report would be published on Wednesday.
"We will continue these investigations so we get an isolation of the disease itself, and an isolation of the cause of the disease," he said.
He said compensation for farmers was being increased, the Inland Revenue was offering assistance and a helpline remained open.
FOOT-AND-MOUTH IN NUMBERS
97 cattle were culled from the first outbreak - 64 from infected premises, 33 from neighbouring premises. 102 were culled in the second outbreak
111,000 farms across UK affected by movement ban
That includes 10m cattle, 23m sheep and 5m pigs
NFU President Peter Kendall said the HSE report was an "important step" in understanding how the virus might have spread.
But he added: "I have spoken to many farmers over the last few days who are absolutely horrified that the source of this outbreak could be from Pirbright.
"And the suggestion in the findings that the disease could have been transferred, either accidentally or deliberately, by humans will only add to these concerns."
Earlier, John Gunner, the second farmer whose herd was culled, said: "I would feel annoyed that people who are dealing with such dangerous diseases are totally irresponsible."
The 60-year-old farmer wept as he spoke of how his cattle began to deteriorate before his eyes, drooling and limping.
Recalling his favourite bull, Ned, he said: "My old bull Ned, he was not very well. He was like having a pet dog, he was about eight years old. His father was one of the winners at Smithfield Market, he had great pedigree, he was so gentle."
About 200 cattle have been culled on the two farms since Friday and the government has banned the movement of all livestock across Britain - although some restrictions have now been relaxed in Scotland.
The European Commission has formalised a ban on British exports of meat, milk products and live animals.
Merial had been involved in "large scale production" of the strain
Environment Secretary Hilary Benn said further inquiries would be carried out into drainage on the Pirbright site and the possibility that the strain had been released by human movement would be investigated further as "a matter of urgency".
Following criticism from some farmers and opposition parties, he also ordered that all footpaths within the 3km protection zone should be closed with immediate effect.
The NFU has estimated the outbreak could cost "tens of millions of pounds", affecting not just farmers but related industries such as abattoirs.
Defra has set up a helpline in response to the latest outbreak on 08459 335577.