SEC in S&P downgrade investigation?

NEW YORK (CNNMoney) -- Securities and Exchange Commission regulators have sent subpoenas to various financial firms as they look into possible insider trading before Standard & Poor's cut the U.S. credit rating last month, according to a report published Tuesday.

The Wall Street Journal said SEC officials are focusing on firms that placed bets on a stock market drop just before S&P downgraded the United States to an AA+ rating from an AAA rating early last month, though it isn't clear which firms have received subpoenas.

SEC representatives declined comment to CNN on the investigation.

Citing an unnamed source, the Wall Street Journal says that recipients of the subpoenas were asked which person at their firm first heard about the ratings agency downgrade, when they were told about it, and by whom.

The report also says that the SEC is working with a self-regulating Wall Street Agency called the Financial Industry Regulatory Authority, which uses high-tech computer software to dig through thousands of daily trade reports on all the major stock exchanges.

FAQ: Behind the downgrade

The S&P decision to downgrade the U.S. credit rating came late on Aug. 5 after the agency decided that the U.S. government fell short in its effort to tackle long-term debt issues.

On Aug. 8 -- when the U.S. market opened for trading the first time since the downgrade -- the Dow Jones industrial average had its worst one-day drop since the 2008 economic crisis, falling 635 points.

-- CNN's Katy Byron contributed to this report.