Former Provisional IRA leader Thomas 'Slab' Murphy was questioned after gardai and PSNI officers in helicopters swooped on his home in one of the biggest ever crackdowns on suspected cross-border fuel laundering.
More than 300 members of the PSNI, Garda, Revenue and Customs, the Irish Defence Forces and the British Army were involved in the raid, which centred on the suspected fuel laundering activities of the former IRA chief-of-staff.
His home, six feet inside the Republic, was surrounded by personnel, many of whom arrived on either side by Air Corps and British Army helicopters.
Murphy (60), who was named under privilege during a Smithwick Tribunal hearing in 2011 as a chief-of-staff of the IRA army council, drove off from his home at Ballybinaby, Co Louth, as the search teams arrived at 5am.
However, he was stopped and questioned at an outer security cordon that had been put in place at 4am. He was not arrested.
Anti-crime agencies raided 22 premises in 11 counties – Louth; Armagh; Monaghan; Dublin; Kildare; Waterford; Offaly; Roscommon; Westmeath, Meath and Tipperary.
Murphy's house and outbuildings were searched.
The home of a petrol retailer and the offices of an international transport company were among the premises raided.
A large number of files, computers and discs were seized during the raids. More than two dozen bank accounts have been frozen and cash seized.
Senior security sources believe they have uncovered one of the country's biggest laundered fuel operations, costing taxpayers north and south of the border millions in lost revenue each year.
Yesterday's raids came after months of investigations by the Republic's Criminal Assets Bureau (CAB), assisted by Customs and Excise in Northern Ireland.
A security source said: "We are confident that enough evidence has been gathered to bring a case before the courts."
A separate case against Murphy, who denies any Provo role, is due to be mentioned at the non-jury Special Criminal Court in Dublin this month.
He faces nine allegations of failing to file income tax returns from 1996 to 2004.
Murphy is taking a Supreme Court case in an effort to have the case against him heard before a jury. Yesterday's operations also focused on two associates.
It is understood that CAB investigators have uncovered several bank accounts in bogus names as a result of the inquiry.
"This investigation has been ongoing for several years," said one source.
"It is just like a jigsaw and the final pieces in that puzzle are now being put in place."
A Garda spokesman confirmed that a large quantity of cash was discovered at two locations that were searched yesterday.
A fuel laundry with the capacity to launder an estimated several hundred thousand litres of diesel was also discovered.
Gardai said the plant had a potential throughput of 10 million litres a year, which equates to a loss to the exchequer of €5.5m.
A senior Garda source described it as "the biggest fuel laundering operation on these islands" after more than 40,000 litres of laundered fuel were found at the site.
Thomas 'Slab' Murphy (60) is a former leader of the Provisional IRA. He lives at Ballybinaby in Co Louth and his home is just six feet inside the Republic's border. Murphy has always denied his role with the Provisional IRA although he was named as chief-of-staff of the terror group's army council during a Smithwick Tribunal hearing in 2011.
He had a fallout with the Provos after the peace process.