Teams of specialist investigators today remain on site at Banham Poultry in Attleborough following the bodies of two men being found at the factory.
Police and safety experts are looking into the refrigeration gas leak in an effort to deter whether it has any link with the men's deaths.
Emergency services were called to Banham Poultry, on Station Road, at about 1.10am on Thursday, following reports two subcontractors working on the site from a pest control company had been found deceased.
A police cordon has been put in place at the scene whilst enquiries continue and this is likely to remain in place for the rest of the day.
It is still believed there is no threat to the wider public as result of the gas leak.
The Health and Safety Executive (HSE) has launched a joint investigation with Norfolk police. A post mortem examination is due to take place in due course to determine the cause of the men's deaths.
The discovery of the bodies in the early hours of yesterday saw fire crews including environmental protection units called to the scene with firefighters using breathing apparatus and gas monitors to make the area safe.
Paramedics and an air ambulance were also scrambled to the factory but efforts to save them the two men died at the scene.
The initial police cordon saw one platform closed at Attleborough Station, which backs on to the factory, with trains heading to Cambridge not stopping. These restrictions were lifted at 5.30pm yesterday and services are running as normal today.
The deaths have shocked other local pest control workers.
Richard Pummell, owner of Express Pest Control, based in Thetford but also covering Attleborough, said: 'It has taken us all by shock and dismay that something like this can happen on our doorstep. There is a lot of speculation about what might have gone wrong but I can't comment on that. But it was an extremely sad day.'
He said working on commercial premise like a factory usually involved very thorough risk assessments.
'You really must stick to product labels and carry out a thorough risk assessment each time you conduct any work,' he said. 'In that way you do reduce the risk of injuries or fatality in this case.
'Nobody knows what has happened yet. Obviously it is a worry to all pest controllers not only locally but nationally and we will be interested to see what went wrong. This can be a very hazardous occupation.'
The deaths came on the same day that Banham Poultry announced its future could be at risk with the loss of up to 1,000 jobs. Based in Attleborough, the plant where processing takes place is one of the town's major employers.
The company said it was deeply saddened by the incident and was working closely with the police and health and safety authorities to determine what happened.
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