Eleven people have been arrested in England and Northern Ireland over an alleged £10m fraud connected to the trade in reduced-duty fuel oils.
Red diesel is chemically marked to discourage its trade
Revenue and Customs officers raided homes and businesses in counties Londonderry and Armagh, Cheshire, Essex, north London and Hertfordshire.
The inquiry concerns kerosene and the fuel known as red diesel, used in agriculture and construction vehicles.
Eight men and three women have been released on police bail until January.
A spokeswoman for HM Revenue and Customs (HMRC) said its investigation focused on the alleged evasion of excise duties and VAT and money laundering.
Anyone trading in rebated oils must register with HMRC. Red diesel is chemically marked and dyed red to discourage its trade and use by ordinary motorists.