Gardai now believe that foot-and-mouth may have been brought into the Republic by calves illegally imported by a suspected smuggler who lives in the Border area.
These calves have not been located, and the potential for infection elsewhere in the Republic remains.
Gardai, members of the National Bureau of Criminal Investigation and a special investigation unit from the Department of Agriculture are trying to trace all movements of illegally imported cattle and sheep in the crucial days before the outbreak in Meigh, Co Armagh, was confirmed.
Sources have told gardai that a batch of 38 calves was taken illegally across the Border and delivered to farms in Leinster and/or Munster. Results of tests on the truck used to transport the calves are due this week.
Dealers in the Republic who have been arrested have not revealed the final destination of the 38 calves. One garda said: "It is a miracle FMD is not widespread throughout the country."
The suspected smuggler, who was arrested and questioned by detectives, is said to have left the jurisdiction on his release from custody. He was among the first few people interviewed by gardai and Department of Agriculture officials investigating the Proleek outbreak.
The IFA in Louth has praised the work of gardai in establishing the source of the outbreak. The Louth IFA chairman, Mr Raymond O'Malley, said it was now necessary to appoint a permanent special investigation unit for the Border area, particularly in Louth and Meath, to tackle the activities of rogue dealers and smugglers.