Thirteen men and a woman were arrested yesterday by revenue and customs officers investigating a suspected £15 million money laundering racket by an organised crime gang.
Nine men and the woman were held in Belfast, with the three other men arrested in Derry, Armagh and Antrim.
A 14th man was detained in London.
The investigation is connected to income tax and VAT fraud linked to the construction industry.
The HMRC described the raids as the largest operation of its kind ever undertaken in Northern Ireland.
Assisted by officers from the Police Service of Northern Ireland, revenue staff searched 31 business and residential properties in Belfast, Armagh, Antrim and Derry.
In co-ordinated activity in Stoke Newington in London, one man was arrested after two businesses and one private address were raided by customs officials accompanied by officers from the Metropolitan Police.
The 14 were being interviewed by revenue officials and investigations were continuing.