SEC Enforcement Director Robert Khuzami's comments to Reuters came a day after McGraw-Hill Cos Inc disclosed on Monday that the agency may sue its Standard & Poor's unit for breaking securities laws.

Khuzami declined to comment specifically on S&P, but his remarks show how a case against S&P or other credit raters would be far from easy to win.

"There are some statutory challenges in the law, and some disclosure-related challenges that are unique to credit-rating agencies that can make the cases more challenging," Khuzami said.