A man has died during a raid by police investigating the abduction of Kevin Lunney.
Cyril McGuineess, living at an address in Derbyshire, is believed to have been one of the main suspects in the investigation.
Derbyshire Police said their officers carried out a search warrant at an address in Buxton on Friday morning, on behalf of the Police Service of Northern Ireland (PSNI).
Shortly after 7.30am, a 54-year-old man was taken ill.
Despite first aid being administered by officers, he was later pronounced dead.
The incident has been referred to the UK's Independent Office of Police Conduct (IOPC), which has launched an investigation.
Police investigating the assault and abduction of the Quinn Industrial Holdings (QIH) executive carried out major searches on both sides of the border earlier.
Gardaí searched 12 locations south of the border while the PSNI searched five locations in the North.
Meanwhile, British police were searching the home in Derbyshire.
Over 100 officers from a range of Garda units were involved.
An Garda Síochána said they had competed all the searches, which began on Friday morning in Cavan, Longford and Dublin.
They said the searches were a "valuable step" in the evidence gathering stage of the ongoing investigation.
However the force is not providing any further information.
An Garda Síochána continues to appeal to any person who has any information, no matter how insignificant it appears, on this attack or any other criminal activities which have taken place in the Cavan/Fermanagh border area over recent years to contact An Garda Síochána at Cavan Garda station om 049-4368800, the Garda Confidential Line at 1800-666-111, Crime Stoppers on 1800-25-00-25 or any Garda station.
Additional reporting: Jack Quann