More than a quarter of a million pounds has been illegally plundered from a ground-improvement scheme involving up to 30 sports clubs.
GAA and Irish league clubs are among those under investigation.
Accounts have been examined after clubs were awarded special British government grants to upgrade facilities.
A source close to the investigation said: "At the minute the fraud looks like it is in excess of £250,000. But it's still unclear how big it's going to be".
Under the Safe Sports Grounds Scheme, clubs were entitled to 80 per cent funding for urgent stadium repairs, with the rest coming out of their own finances.
Detectives were called in by the Northern Ireland Sports Council when irregularities were detected during monitoring checks.
Half of the 59 soccer, rugby and Gaelic Athletic Association clubs involved in the project, which paid out £3.3 million between 2000 and 200,2 are being investigated.
The inquiry is centred on invoices for work carried out under the scheme.
The investigation is expected to take several months to complete and files may then be submitted to the Director of Public Prosecutions.
PA