Regulators have opened an investigation into KPMG's auditing of Fannie Mae, the mortgage finance company which faces a possible $9bn restatement after violating US accounting rules.

The Public Company Accounting Oversight Board, which regulates auditors of listed companies, decided on Tuesday to authorise its enforcement staff to scrutinise KPMG's work. The inquiry adds to investigations by the Department of Justice and the Securities and Exchange Commission into Fannie's activities.