Detectives will be joined by a team of specialist officers to conduct the investigation, which is now expected to run for at least two years.
The initial inquiry began in March after a district auditor's management letter revealed several irregularities.
The auditor, Gordon Sutton, said there was 'a culture which has grown up which needs to be looked at'. This included frequent trips by members to exotic overseas destinations on club and business class flights and a free bar at the council-run Doncaster Race Course.
The auditor believes an agreement whereby Keepmoat gets first refusal on all surplus council land could be illegal.
In his report the auditor said: 'There are serious concerns over the lack of control in some sensitive areas.'
Superintendent Graham Johnson, leading the inquiry, said there had been a 'considerable amount of rumour and speculation' and until he had papers from the auditor and fraud squad he would not have a firm idea of the scope of the investigation.
A spokesman for Doncaster MBC said: 'We have co-operated with the district auditor's inquiry and we will continue to co-operate with the police.'