Four partners at the Belfast office of KPMG have been arrested in connection with suspected tax evasion.
HMRC officials visited the global accountancy firm's city centre office on Wednesday and detained the men.
They are Jon D'Arcy, Eamonn Donaghy, Arthur O'Brien and Paul Hollway, the firm's most senior staff in Northern Ireland.
KPMG said it is cooperating with the investigation and the four men have been placed on "administrative leave".
The firm added that it does not have "any indication that this investigation relates to the business of KPMG or the business of our clients".
Mr Donaghy is KPMG's head of tax in Belfast and has been heavily involved in the campaign to have corporation tax powers devolved to the Northern Ireland Executive.
Mr Hollway is the firm's head of corporate finance in Ireland.
Aside from their KPMG roles, the four men are directors in a property investment company called JEAP Ltd.
It made heavy losses when the property market crashed in 2008.
It is not clear if that forms part of the HMRC investigation.