The Securities and Exchange Commission is scrutinizing the method Standard & Poor's used to cut the U.S.'s credit rating and whether the firm properly protected the confidential decision, according to a person with direct knowledge of the matter.

SEC inspectors are examining S&P's policies for conducting such analyses and whether those procedures were followed when the New York-based firm downgraded the U.S.'s credit rating Aug. 5, said the person, who declined to be identified because the inquiry isn't public.

S&P's downgrade of the U.S. contributed to an equity rout that erased about $6.8 trillion from global stocks since late July. U.S. officials have said the downgrade was based on a flawed analysis that overstated the nation's debt by about $2 trillion, while S&P said the discrepancy doesn't change projections that the U.S. debt-to-gross domestic product ratio will probably continue to rise in the next decade.