Investigators are studying claims that European Commission cleaning contracts were faked in a four-year fraud potentially worth millions of euros.
The allegations come three months after another scandal over contracts
The probe is headed by the Brussels prosecutor and the EU anti-fraud unit, Olaf, which said contracts may have been "executed in an irregular manner".
Olaf said the investigation was focusing on individuals and companies outside the EU institutions
However, media reports suggested bureaucrats had also been involved.
"The financial impact of the suspected irregularities cannot be assessed at this stage," Olaf spokesman Alessandro Buttice said in a statement.
"The total volume of the contract in question is 44m euros ($59m; £30m) covering a period between 2003 and 2007.
"As the cleaning services have in fact been rendered, any potential damage could only amount to a fraction of this."
Commission spokesman Maximilian Strotmann said it was unclear how much money was involved.
Olaf launched its investigation in 2005 and referred the case to the Belgian federal prosecutor on 1 March.
Belgian media alleged that European Commission employees had provided false contracts for cleaning services never carried out.
Saturday's Le Soir newspaper said the whistle was blown by an accountant with the cleaning company who was fired after he pointed out that figures did not tally with work carried out by the firm.
Olaf said it launched its investigation on the basis of information received from the European Commission and from external sources.
The allegations come three months after three Italians, including a European Commission official, were arrested in a case of suspected corruption involving tenders of building contracts.