Detectives were last night interviewing a haulier who had driven his lorry from a foot-and-mouth-infected abattoir in Britain back to Northern Ireland and delivered animals to the Republic.
Meanwhile, a farm to which the haulier is understood to have delivered cattle at the weekend has been sealed off in an area between Mullagh and Virginia, Co Cavan, by gardai and officials from the Department of Agriculture.
The Minister for Agriculture, Mr Walsh, said yesterday he did not want investigations into this lorry to be seen as the sole or primary focus of efforts to keep out the disease.
However, a direct link to a confirmed infected site in Britain has caused near panic in the North and the Republic, and officials are working closely to track the vehicle's movements.
The Northern authorities said yesterday that sheep transported back to Northern Ireland in a lorry had been slaughtered in a Northern meat factory.
Official spokesmen on both sides of the Border were stressing last night that because the haulier was being interviewed by the police did not necessarily imply any kind of wrongdoing.
Meanwhile, the build-up of Garda personnel at the Border to prevent the importation of live animals or animal products continued, and by late today over 200 crossings will be guarded.
The Army has been called in to assist in the operation, and inspections have been strengthened at ports, airports, meat plants and farms.
The Department has also started a publicity campaign in national newspapers and has cancelled all its public meetings to prevent the possible spread of the disease. The IFA has cancelled all its public events and has said it has done so to minimise the risk of spread.
A midlands meat company, Glanbia, of Clara, Co Offaly, has put its workers on short time because of the crisis, it was learned last night.
A number of co-operatives operating on both sides of the Border were in difficulty yesterday with the ban on the import of milk from the North. Town of Monaghan, where the majority of suppliers are from Northern Ireland, said it was working with the Department to resolve the issue.
Meanwhile, British farmers - still reeling from the BSE crisis, and now fearing fresh disaster - have been warned they may have to wait at least a week before the extent of the outbreak is known.
The warning came as the Agriculture Minister, Mr Nick Brown, held crisis talks with farmers' leaders in London and as a second suspect abattoir, in Guildford in Surrey, received the all-clear.
With the authorities preparing to visit hundreds of farms across Britain, Mr Brown said they were "acting rigorously" and no new cases had been reported so far.