Three men from Wolverhampton have been arrested on suspicion of involvement in an £800,000 fuel laundering operation.
Officers from HM Revenue and Customs (HMRC) and West Midlands Police undertook searches at two domestic premises and an industrial unit in the city on May 22.
The ongoing investigation centres on the purchase of rebated kerosene and its sale to motorists as duty-paid road diesel.
Around 1,500 litres of kerosene was removed from the industrial site, along with storage tanks, a fuel pump and 8,000 fake Viagra tablets. Eight kerosene-fuelled vehicles were seized from drivers.
The three arrested men were questioned and released on bail until August. Investigations are continuing.
Colin Booker, assistant director at HMRC, warned that kerosene can damage engines and said: "Anyone with information about fuel fraud can contact the Customs Hotline on 0800 595000."