The CEO of Quinn Industrial Holdings has said the latest threats against its directors are "being taken very seriously at the highest level".
Liam McCaffrey said the PSNI and gardaí are monitoring the homes and places of work of QIH's directors.
The threat, delivered to the Irish News, was conveyed to Quinn directors by gardaí.
In the letter, which was typed and delivered to the Irish News on Monday, QIH directors were told that it was their final warning.
Speaking on RTÉ's Morning Ireland, Mr McCaffrey said he was at home on Monday evening watching television when a PSNI officer came to his house to inform him of the latest threat.
Mr McCaffrey said it was a simple matter of criminality and not a community issue, and those under threat had to trust in the ongoing investigations to resolve the matter.
He added that the amount of support the directors have received locally "has been very heartening".
Mr McCaffrey said he understands that a number of people have come forward and he urged anyone with information to approach gardaí or the PSNI.
Leo Varadkar said "the rule of law" must prevail, that the Government had increased garda resources along the border and would do so again.
- Quinn Industrial Holdings directors receive 'last warning' death threat
- Quinn director describes attack on Lunney as barbaric
Mr McCaffrey said that he and his fellow directors have stuck with the job for the last number of years and want to "see the job through".
However, he added, the directors all have young families and they "must balance the risks that are involved".
"We need people to come forward. A lot of people have come forward, I believe, to the garda over the last number of weeks and we now need to see that investigation brought to a conclusion," he said.
Mr McCaffrey added: "We have been the subject of this intimidation for the last four years now and we have stuck with the task. Certainly it's our intention to see things through."
The anonymous authors told the directors that if they wanted, "we could have killed Kevin very easily", in reference to the attack on Quinn director Kevin Lunney near his Co Fermanagh home last month.
Mr McCaffrey said that Mr Lunney is recovering well. He said returned to work yesterday for an hour and received a round of applause from the staff and it was very emotional.
He said Mr Lunney was strong and determined and knows that he was supported by staff at QIH and the local community.
The directors were told in the letter they had not learned their lessons after what happened to Mr Lunney.
It also threatened Cavan County Council staff or other contractors who remove posters erected as part of the campaign of intimidation against the directors.
It warned that they will be targeted if they remove the posters.
The five directors are Mr McCaffrey, Mr Lunney, Tony Lunney, Dara O'Reilly and John McCartin.
The Taoiseach has said the Government is treating recent 'lawlessness' in border counties with the utmost seriousness and is keen to see law and order restored pic.twitter.com/hJuuZg5avL— RTÉ News (@rtenews) October 30, 2019
Earlier, Assistant Garda Commissioner for the North West Region Barry O'Brien said that despite the latest threat, he was confident of progress in the ongoing investigation.
Assistant Commissioner O'Brien said gardaí have in the past been very successful in tackling criminality in the border region and a substantial investigation was now under way.
He said protective measures have been put in place both in the Republic and across the border for the directors of QIH.
On the investigations into the arson attack on Sinn Féin TD Martin Kenny's car outside his home in Co Leitrim and an arson attack on Emyvale Garda Station in Monaghan, Mr O'Brien said all efforts will be made to bring the perpetrators to justice.
Garda Commissioner Drew Harris will meet the Directors of Quinn Industrial Holdings on Tuesday.