Health inspectors are investigating the possibility that foot-and-mouth was transferred to a farm in Surrey by employees of a nearby research site.
The disease has been identified at two farms in Surrey
One line of inquiry is that workers at vaccine manufacturer Merial, at Pirbright, could have picked up the infection because of drainage problems.
Allotments near the farm where the outbreak began are also being investigated.
Merial said there was no evidence the virus was spread by humans.
The first cases of foot-and-mouth were found in cattle at Woolford Farm, near Guildford, on Friday.
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
A second outbreak was confirmed at a farm on Monday, and a total of 214 animals have been culled since Saturday.
On Monday, the European Commission formalised a ban on British exports of meat, milk products and live animals.
And the movement of all livestock across Britain has also been banned - although some restrictions have now been relaxed in Scotland. An estimated 110,000 farms are affected.
Traders at London's historic Smithfield Market said meat supplies at the market were already starting to dry up.
However, the government is expected to relax the movement ban to allow farmers in England and Wales outside the Surrey surveillance zone to take animals for slaughter.
The Department for Environment, Food and Rural Affairs (Defra) is also considering licensing the movement of fallen stock - animals which die on farms - on a farm-by-farm basis.
The move follows complaints from farmers that they were unable to dispose of dead livestock.
Defra said "all lines of inquiry" into the cause of the outbreak were being looked at.
The Health and Safety Executive says there is a "strong probability" the outbreak began at the Pirbright research site, home to Merial and the state-run Institute for Animal Health and only a few miles from the farms.
But it did not specify which of the two facilities was to blame.
The executive said there was a "negligible" risk the virus had been spread by the wind or flooding, but it could have been the result of human movement or "accidental or deliberate transfer".
'Confidence' in processes
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.
It is understood investigations have discovered a link between problems with drainage and the possible actions, accidental or deliberate, of Merial employees who may use the nearby allotments.
Merial said it had conducted "intensive internal investigations" and had "complete confidence" in its processes and procedures.
In a later statement the company confirmed one of its employees had accompanied investigators to an allotment but said there was "no evidence linking this member of staff to the outbreak".
Prime Minister Gordon Brown said he hoped a second report would be published on Wednesday.
The National Farmers' Union is considering legal action "if it turns out to be a commercial company that has been and can be shown to have been careless in any way" which could claim millions of pounds, president Peter Kendall says.
EU veterinary and food safety experts will meet in Brussels later to formally assess the UK government's response to the foot-and-mouth outbreak.
The commission has already said it is satisfied the government is doing everything it can.
Following criticism from some farmers and opposition parties, all footpaths within the 3km (1.8 mile) protection zone have been closed.
But shadow minister for culture, media and sport, Tobias Ellwood, said this action had been taken too late and that he had wandered around on Tuesday at allotments near the outbreak site.
Defra has set up a helpline in response to the latest outbreak on 08459 335577.