Two more men have been arrested by police in the Republic of Ireland in connection with the abduction of County Fermanagh businessman Kevin Lunney.
Mr Lunney, a director at the Irish firm Quinn Industrial Holdings (QIH) was kidnapped and tortured before being dumped on a roadside on 17 September.
Four men have now been arrested on Thursday over the abduction.
A 25-year-old man was arrested by gardai (Irish police) in Dublin and a 66-year-old man in County Cavan.
Earlier, two other men, both aged 38, were detained in Dublin.
Gardai said they can be held for seven days before being charged or released.
A man in his 20s, a man in his 40s and a woman in her 50s were detained by gardaí last Thursday after searches in counties Cavan, Longford and Dublin.
They were released in the early hours of Saturday 16 November and gardaí said a file would be sent to the public prosecutions director.
The attack on Mr Lunney is the subject of a major cross-border investigation by gardaí and the Police Service of Northern Ireland (PSNI).
Almost a fortnight ago, the main suspect in the investigation, Cyril McGuinness, died during a police raid at his home in Derbyshire, England.
Mr McGuinness collapsed as officers from Derbyshire Police searched his property at the request of the PSNI.
An ongoing inquest in to his death was told he had been suffering from heart disease and died of a suspected "cardiac event".
Mr Lunney was abducted by a gang outside his family home in Kinawley in County Fermanagh as he drove back from work.
The businessman, a father of six, sustained a broken leg, was slashed with a knife and doused with bleach in a two-and-a-half hour ordeal.
The letters QIH were also cut into his chest.
Mr Lunney and other directors of QIH have been subjected to a long-running campaign of intimidation and arson attacks.
A statement from gardaí said they continued to appeal for 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".