A spokeswoman for the Special Inspector General overseeing the financial bailout says his office is looking to recent admissions by the company, which received $16.3 billion in government bailout money.
GMAC has halted foreclosures in 23 states.
Taxpayers own well more than half of the company. The Treasury Department first propped up GMAC Mortgage parent Ally Financial as part of the auto industry bailouts. The money might not be returned.
Treasury officials have said they did not scrutinize GMAC's treatment of mortgage borrowers. They did not want to micromanage the company.
GMAC's admission touched off a flurry of foreclosure halts by big banks with similar problems.