One of the men suspected of the abduction, assault and torture of Northern Ireland businessman Kevin Lunney died while police raided his home in England on Thursday.
The man took ill during the raid on his home in Derbyshire, England and subsequently died.
PA understands the man who died was convicted criminal and crime gang member Cyril McGuinness.
McGuinness, originally from the Derrylin area, went by the nickname “Dublin Jimmy”.
It is understood he was being treated as a chief suspect in the investigation into Mr Lunney’s abduction.
It is thought McGuinness, who was well known to the authorities on both sides of the border, was “lying low” in England amid the intense public and police focus on the crime.
Mr. Lunney was abducted, assaulted and tortured by a gang last month.
Mr. Lunney is a director of Quinn Industrial Holdings (QIH) in County Fermanagh was abducted close to his home in September.
The gang took Mr. Lunney across the border where they viciously assaulted him, breaking one of his legs, slicing his face with a blade and carving the letters QIH on his chest before pouring bleach over him.
The attack came amidst five years of coordinated intimidation of businesses which own companies formerly run by Irish business tycoon, Sean Quinn.
The Quinn family have consistently condemned and distanced themselves from those attacking the new owners.
Commenting on the PSNI operation, Detective Chief Inspector Julie Mullan said: "The investigation is continuing at pace and today's significant operation involves searching four residential properties and one commercial premises in the Derrylin area."
She added: "This was a truly horrific crime and we continue to work closely with our colleagues in An Garda Siochana and now also Derbyshire Constabulary to try and bring the perpetrators to justice."