A large amount of documentation, laptops and other evidence has been found at a property in which the main suspect behind the campaign of intimidation and violence against senior Quinn executives died during police raids in the UK on Friday.

Security sources told The Irish Times the material found in the house in Derbyshire “looks like a treasure trove” of information that can aid the Garda and PSNI investigation into the campaign of intimidation against the executives at Quinn Industrial Holdings (QIH).

The same sources said the documents and data on electronic devices would be analysed closely in an effort to establish if any of it links the deceased suspect to a person QIH executives have described as “the paymaster” in funding the campaign against them.

A series of raids in the Republic, Northern Ireland and in Derbyshire against the gang suspected of engaging in the violence was carried out on Friday. And sources familiar with the operation say the gang leader who died in Derbyshire, Cyril McGuinness aka Dublin Jimmy, believed he was in a safe house that was unknown to law enforcement in Ireland or Britain.

“It seems at this stage that he was absolutely shocked he was found and when his door was knocked in he went into cardiac arrest and died,” said one source. Although the same person stressed only a postmortem would establish the cause of death.

Another source said because the dead man – who is originally from Dublin but has lived in the Border region, mainly Fermanagh, for years – believed his location was unknown and that he would not be found. The documents and devices he had taken with him were likely very significant and may “blow the whole case open”.

A spokesman for Derbyshire Constabulary confirmed the death of a 54-year-old man in the raid, but would not disclose his identity or nationality.

Derbyshire police said their officers carried out a search at an address in Buxton at 7.30am on Friday on behalf of the Police Service of Northern Ireland and that a man inside the property was taken ill a short time later.

“Despite first aid being administered by our officers, the 54-year-old man was later pronounced dead,” said the British police force.

“We have referred ourselves to the Independent Office of Police Conduct, which has launched an investigation into the circumstances.”

The office is routinely contacted where an individual dies following police contact, a spokesman for Derbyshire Constabulary said.

He said the Independent Office of Police Conduct, which oversees a system for handling complaints against police in England and Wales, would be carrying out an independent investigation into the circumstances around the death.

This was a routine procedure that was followed where an individual dies following police contact, he said. The spokesman said that a court inquest would be held into the death of the man at a later date.

McGuinness, in his 50s, with an address on Teemore Road, Derrylin, Co Fermanagh, was the main suspect behind the campaign of intimidation and violence against senior Quinn executives in the Border region.

He is believed to have been the head of the gang that has been behind the violence, including the abduction and torture of Kevin Lunney in September.

Several co-ordinated raids

Gardaí said late on Friday afternoon that it had completed all searches which started this morning in Cavan, Longford and Dublin.

Garda Commissioner Drew Harris and Assistant Chief Constable Mark Hamilton of the PSNI brief the media at Garda HQ about the joint operation. Photograph: Gareth Chaney/Collins

“These searches were a valuable step in the evidence-gathering stage of this ongoing criminal investigation,” said the force.

Gardaí had been searching five locations in Co Cavan, three in Co Longford and four in Dublin. The premises were a mixture of domestic dwellings and commercial/business premises.

The searches were part of the evidence gathering stage to progress the investigations into the abduction, false imprisonment and assault of Mr Lunney (50) on September 17th.

More than 100 members of the force are assisting the investigation team at Cavan Garda station including colleagues from Cavan-Monaghan Garda Division, regional support units, national support service, dog units, National Bureau of Criminal Investigation, Drugs and Organised Crime Bureau and National Public Order Unit.

The PSNI were searching five locations in the Derrylin area in Co Fermanagh and Derbyshire Constabulary are searching one location in Britain.

Detective Chief Inspector of the PSNI Julie Mullan said the investigation is “continuing at pace” .

“Today‘s significant operation involves searching four residential properties and one commercial premises in the Derrylin area,” said Ms Mullan.

“This was a truly horrific crime and we continue to work closely with our colleagues in An Garda Síochána and now also Derbyshire Constabulary to try and bring the perpetrators to justice.”

What happened to Kevin Lunney?

On Tuesday, Mr Lunney, who is a senior executive of QIH, told of how he was abducted and tortured by the gang in mid-September.

Kevin Lunney speaking about his ordeal on the BBC Spotlight programme.

In an interview with BBC Spotlight, Mr Lunney recalled how a car parked near his home smashed into his Jeep and how the gang broke the windows of his vehicle and dragged him outside.

One of the assailants held a Stanley knife to his neck and shouted: “Get into that. If you don’t get into that we are going to kill you!”

Mr Lunney described how the gang drove away from his home, tortured him for two hours before later dumping him semi-naked on a Co Cavan roadside.

He said the gang broke his leg in two places, cut his face and carved the initials QIH into his chest with a knife on orders to force him to resign.

Mr Lunney believed his captors were acting on a “list of brutally specific orders”, he added.

The Garda said it continues to appeal to any person who has any information, no matter how insignificant it appears, on this “vicious criminal attack” or any other criminal activities which have taken place in the Cavan-Fermanagh Border area over recent years to contact them at at Cavan Garda station 049-4368800, the Garda Confidential Line 1800-666 111, or Crime Stoppers on 1800-250025 or any Garda station.

QIH was once owned by local businessman and former billionaire Seán Quinn who lost his family business over multibillion losses on a boom-time bet on Anglo Irish Bank shares.

Mr Lunney as well as Liam McCaffrey and Dara O’Reilly were among a local management group who in 2014 took over running the Co Fermanagh-based building materials group founded by Mr Quinn in the 1970s. They did so on behalf of a consortium of three US hedge funds which had acquired the group’s borrowings in the debt markets.

They are among a group of QIH managers who received deaths threats demanding they resign. The warnings follow years of intimidation which the new management team endured.

Mr Quinn has always denied being behind the campaign and has condemned the latest acts of violence and intimidation.

Meanwhile, Minister for Justice Charlie Flanagan welcomed the establishment by the Garda and PSNI of a joint investigation team to address the attacks on QIH and its management.

“This is a significant step forward and I commend the Garda Commissioner and his counterparts in PSNI management for taking this initiative which has been in preparation for some time,” said Mr Flanagan.

The joint investigation team was established under article 20 of the second additional protocol to the European Convention on Mutual Legal Assistance in Criminal Matters. This is the first joint investigation team in which the Garda has taken part.