Further tests have again found horse DNA in burgers at the factory at the centre of the horse meat scandal.
Nine out of 13 burgers tested positive for traces of horse DNA in new tests carried out this week.
The tests also firmly pointed the finger of blame at an ingredient that had been imported from abroad.
Production was shut down at the Silvercrest meat plant in Ballybay, Co Monaghan, after the department released preliminary details of tests carried out there on Tuesday.
The Republic's Department of Agriculture said it had informed Silvercrest of the further laboratory test results, and the company later said it would temporarily shut down all production at the plant until it had completed its investigation.
The company said it believed it had now identified a third party EU supplier as the source of the horse-meat contamination.
Products produced at the plant this week have not been released into the market. Agriculture Minister Simon Coveney welcomed the decision by the company.
As well as tests on finished burgers, the department also tested seven samples of raw ingredients from Silvercrest that went into making them.
Only one of these tested positive for horse DNA, and that was an ingredient sourced from abroad. All the ingredients sourced from Irish suppliers tested negative for equine DNA.
Test results are not yet available for samples taken from Liffey Meats in Ballyjamesduff, Co Cavan, on Tuesday.
The department did not quantify the exact amount of horse DNA present in the burgers at Silvercrest. The positive samples will have to be further analysed in Germany to determine this.
Mr Coveney said: "The investigation will continue to conclusively establish the source of the equine DNA. The focus of the investigation is now to establish a common ingredient used in the manufacture of burgers in all three plants and from where it was sourced."
The development follows revelations earlier this week that 10 out of 27 frozen burgers tested by the FSAI contained horse DNA, with one Tesco product containing 29pc horse meat.
Silvercrest owners ABP said last night they were suspending all production at the plant.
"Following receipt of this evening's Irish Department of Agriculture results, we believe that we have established the source of the contaminated material to one of these suppliers," ABP said in a statement.
"However, because equine DNA has been found in finished products tested this week, we have decided that the responsible course of action is to suspend all production at the Silvercrest plant in Monaghan with immediate effect."
ABP said all staff would continue to be paid during the suspension, and they would work with the relevant authorities, management and supervisory team on the investigation.
Irish Cattle and Sheep Farmers Association (ICSA) president Gabriel Gilmartin said it was significant that the only raw material sample which tested positive was from another country.
"It looks increasingly like the problem is linked to imported raw materials," he said. "We need answers on why there is any need for imported ingredients in burgers when Ireland is the biggest exporter of beef in the Northern hemisphere."