A suspect in the investigation into the kidnapping and torture of Co Fermanagh businessman Kevin Lunney has died after he took ill during a police raid in Derbyshire, PA news agency sources have said.
Police had launched a co-ordinated search operation on both sides of the border and in England.
At a press conference in Dublin, PSNI Assistant Chief Constable Mark Hamilton described the death as "regrettable".
"The PSNI commissioned a search in England with the support of the police officers there and a report has been that a man has died, which is very regrettable," he said.
"That is now a matter of investigation in England separate to this ongoing inquiry."
Mr Hamilton declined to be drawn when asked whether the dead man was a main suspect in the investigation.
Mr Hamilton was speaking at Garda headquarters with Garda Commissioner Drew Harris to announce that police and prosecutors on both sides of the border had agreed to establish a landmark "joint investigation" into the intimidation campaign.
Mr Harris described the agreement, which will be overseen by EU law and order agency Eurojust, was an "important advance".
"These (attacks) have been obviously a subject of public concern lately and we want to be seen to act in a strong and robust way to show that there's no room for criminality anywhere on the island of Ireland and that as police services we are cooperating in the strongest fashion possible to bring the perpetrators to justice," he said.
On the reported death in Derbyshire, Mr Harris added: "That is our understanding, but that matter happened within a search in England and that is being pursued by the authorities there."
Asked if the joint investigation would be targeting the "paymaster" behind the intimidation campaign, Mr Hamilton urged "patience".
He said this morning;s searches represented "one phase" of the probe and there would be "other phases to come".
Mr Hamilton added: "We have made a significant effort to try to make sure the people of Fermanagh feel safe during this period and we are working very collaboratively on both sides of the border to do that."
Similarly, Mr Harris said gardai were committed to keeping the people of Co Cavan safe.
"These are well resourced investigations, they are proceeding, they are making progress," he said.
"At the same time we have made a commitment to the policing of Co Cavan and to assure the people of Co Cavan that the rule of law will prevail."
Kevin Lunney, a director at Quinn Industrial Holdings (QIH), was abducted outside his Co Fermanagh home in September and taken to a horsebox across the border, where he was brutally beaten.
His attackers broke his leg, sliced his fingernails and face with a Stanley knife, carved QIH on his chest, and doused him in bleach.
Mr Lunney, a 50-year-old father of six, spoke publicly about his ordeal for the first time this week in a powerful TV interview.
Read More: Roots to fallout which saw Sean Quinn leave role at Quinn Industrial Holdings
The attack was the most serious in a five-year campaign of intimidation targeting the companies and directors that now control the business portfolio which was built up by fallen tycoon Sean Quinn, once Ireland's richest man.
A sinister element in the community in the Fermanagh/Cavan border area, where the companies are based, continues to vent anger at the demise of Mr Quinn's empire.
The Quinn family have consistently condemned and distanced themselves from those attacking the new owners.
Read More: QIH directors rule out meeting with Sean Quinn family
Police on both sides of the border have been under mounting pressure to bring the perpetrators to justice.
This morning gardai were searching five locations in County Cavan, three locations in County Longford and four in Dublin. The premises are a mixture of domestic dwellings and commercial business premises.
The PSNI were searching five locations in Derrylin, Co Fermanagh, and Derbyshire Police were searching a location in England.
