NEW YORK (CNNMoney) -- Credit rating agency Standard & Poor's downgrade of the U.S. sovereign debt rating on Aug. 5, which plunged stocks into a roller-coaster frenzy for the past week, didn't come as a surprise.
S&P had warned the downgrade would occur if Congress couldn't agree to comprehensive deficit reduction as part of a deal to raise the debt ceiling.
While S&P, a unit of McGraw-Hill Co. (MHP, Fortune 500), didn't give any public statement about the timing of the move, by the morning of Aug. 5, rumors were rampant on Wall Street that the downgrade would come after the closing bell that day.
Major stock indexes, which started higher on a better-than-expected jobs report that morning, were soon in negative territory, with the Dow Jones industrial average tumbling 240 points before rebounding to close modestly higher.
The rumors turned out to be spot-on correct, as the downgrade was announced just after 8 p.m. ET.
Now the Securities and Exchange Commission apparently wants to know where those rumors came from and how they spread.
The Financial Times reported Friday that the SEC has opened an inquiry, asking S&P to let it know who knew of the downgrade decision before it was announced, as a preliminary look into a possible insider trading inquiry.
The paper, citing an anonymous source, said the agency is not aware of a leak from any S&P insider or any aberrational trading that took place.
The SEC declined to comment on the report. S&P would not comment on any potential probe by the SEC, but said it had taken steps to keep the downgrade confidential before its release.
"S&P takes its confidential information and securities trading policies, and the related securities regulation, very seriously," said the statement. "Our policies prohibit analysts or rating committee members from trading and holding securities or options of the companies or governments they rate. In addition, we have long standing policies and procedures in place regarding the appropriate handling, use and protection of confidential information."
It wasn't just officials within S&P who knew of the pending downgrade. Earlier that day, the Treasury Department was notified by the credit agency that it intended to downgrade the nation's credit rating. Treasury officials challenged that decision, arguing S&P's budget analysis included a $2 trillion mistake.
A senior administration official told CNN on background late on the afternoon of Aug. 5 that S&P was reconsidering its downgrade opinion. But while the credit rating agency changed its numbers to conform with Treasury's, it went ahead with the downgrade anyway.
Some market analysts have questioned whether the downgrade was the major cause of market volatility this past week, since demand for U.S. Treasuries remained strong throughout the week. But others have argued the downgrade added to uncertainty and fed into fears that the United States is at an increased risk of a double-dip recession. 
