Two federal agencies have acknowledged that they are investigating whether the nation’s top credit rating companies committed fraud in the run-up to the financial crisis. 

Officials from the Securities and Exchange Commission’s enforcement division and the U.S. Justice Department said at a Senate Judiciary Committee hearing Wednesday that the raters are under examination for awarding inflated grades to toxic investments at the center of the financial crisis. The raters are already facing investigations from state attorneys general in California, Ohio and Connecticut.

But the federal officials admitted that their examinations face an uphill battle. Robert Khuzami, director of the SEC division, said it “can be difficult to show” the raters knowingly issued bogus ratings—the standard for fraud. Assistant Attorney General Lanny Breuer noted that the raters also enjoy protections from the First Amendment. The raters argue that their ratings are merely opinions, protected by the constitutional right to free speech.

The Investigative Fund recently published a three-part series detailing the raters’ use of the First Amendment as a shield against oversight from the SEC, Congress and the courts. With help from the First Amendment, the series reported, the raters remain undefeated in court.