Five people from Warwickshire have been arrested on suspicion of helping to launder £425,000 which police think was stolen from an electricity company.
Four men and a woman, all of Leamington Spa, were arrested on Thursday by officers investigating an alleged theft by an employee of the National Grid.
The worker, 52, was also detained in Coventry during the raids, say police.
Police believe he passed money through the bank accounts of the five, who were allegedly rewarded for helping him.
All six were arrested on suspicion of theft and deception.
Det Insp Wallis of the Economic Crime Unit said: "We are investigating an increasing number of serious economic crimes where people are allowing their bank accounts to be used to money launder stolen funds, whether free of charge for friends, or receiving financial payment in return.
"Anyone asked to put money through their bank account must seriously question why the person making the request isn't putting it through their own accounts.
"Known as 'farming', this action is essentially money laundering, a criminal activity which can lead to arrest, the search of your home and a potential prison sentence of 14 years."
A West Midlands Police spokeswoman said the six people arrested had been released on bail until December.