BRASILIA (Reuters) - Brazilian federal prosecutors are opening a preliminary inquiry into the acquisition of Banco Master's assets by government-controlled bank BRB for a potential "crime against the national financial system," a person with direct knowledge of the matter told Reuters on Tuesday.

The source at the federal prosecutors' office declined to give details of the preliminary probe, which has not been made public. Prosecutors typically have 30 days to decide whether to turn such an inquiry into a criminal investigation.

Representatives for BRB and Banco Master did not immediately respond to requests for comment.

The acquisition, announced last month and still pending central bank approval, sent BRB shares soaring last week but raised questions about some 23 billion reais ($3.8 billion) of Banco Master assets left out of the deal, including financial products associated with the bank's aggressive growth strategy.

($1 = 6.00 reais)