California's attorney general said his office is investigating Wall Street's big credit rating agencies to determine what role they may have played in the collapse of the financial markets.

Attorney General Jerry Brown said he has issued subpoenas to Moody's Investors Service, Standard&Poor's and Fitch Ratings to determine whether they violated state law in “recklessly giving stellar ratings to shaky assets.”

The agencies are crucial financial gatekeepers, and investors depend on their ratings of public companies and securities to gauge risk and make investment decisions.

At the peak of the housing boom, the agencies gave their highest ratings to securities backed by subprime mortgages that later fell apart. The agencies had to downgrade thousands of securities as the value of those investments plummeted, contributing to hundreds of billions of dollars in losses and write-downs at big banks and investment firms.

The rating agencies either ignored or didn't understand the risks, Brown said. He added that the agencies made billions of dollars in revenue from the securities they rated.

Brown gave the agencies an October 19 deadline to respond to the subpoenas to determine whether they did due diligence in the rating process and whether they profited from giving inaccurate ratings to particular securities, among other questions.