Irish police have arrested three people over the abduction and torture of Quinn Industrial Holdings (QIH) director Kevin Lunney.
Two men, one in his 20s and another in his 40s, and a woman in her 50s are in custody after searches in counties Cavan, Longford and Dublin.
The men can be questioned for up to seven days before being charged or released, the woman for three days.
Meanwhile, detectives are still investigating how a key suspect died.
Mr Lunney was kidnapped and attacked on 17 September.
He was dumped on a road in County Cavan, in the Republic of Ireland, 22 miles (35km) from where he was abducted.
Mr Lunney, a father of six, had his leg broken, was slashed with a knife and doused with bleach in a two-and-a-half hour ordeal.
The 50-year-old had the letters QIH cut into his chest with a knife and told the BBC's Spotlight last week that he feared he would never see his wife and children again.
'No evidence of Taser'
A suspect in the investigation into Mr Lunney's attack died last Friday during a police raid in England.
Cyril McGuinness, 54, is thought to have suffered a heart attack as police searched his Derbyshire home.
Investigators said they have yet to establish the cause of his death, despite a post mortem examination that was carried out on Monday.
The Independent Office for Police Conduct said there was "no evidence" a Taser was used, but added that "work is still ongoing to confirm this."
Body camera footage used by the officers in the raid has been gathered as part of the investigation.
That search was part of a joint police operation across the UK and Ireland in which almost 20 properties were raided.
Sources have indicated that none of the three arrested in the latest operation is regarded as being one of the main players in the kidnapping and torture.
The PSNI said it would "continue to work closely" with the Garda Síochána (Irish police) and Derbyshire Constabulary to try to "bring the perpetrators to justice".
Speaking after the arrests, Irish Justice Minister Charlie Flanagan told the Irish Times that police in Northern Ireland and the Republic needed "time and space" to proceed with a "complex investigation".