The investigation began before Standard & Poor’s cut the United States’ AAA credit rating this month, but it is likely to add fuel to the political firestorm that has surrounded that action. Lawmakers and some administration officials have since questioned the agency’s secretive process, its credibility and the competence of its analysts, claiming to have found an error in its debt calculations…

During the boom years, S.& P. and other ratings agencies reaped record profits as they bestowed their highest ratings on bundles of troubled mortgage loans, which made the mortgages appear less risky and thus more valuable. They failed to anticipate the deterioration that would come in the housing market and devastate the financial system…

The people with knowledge of the investigation said it had picked up steam early this summer, well before the debt rating issue reached a high pitch in Washington. Now members of Congress are investigating why S.& P. removed the nation’s AAA rating, which is highly important to financial markets…

Even though the Justice Department has the power to bring criminal charges, witnesses who have been interviewed have been told by investigators that they are pursuing a civil case.