For more than two years, a team of federal prosecutors had their eyes trained on the largest credit rating agency in the world: Standard & Poor’s Financial Services LLC.

The U.S. Justice Department dedicated nearly two dozen attorneys in Washington and Los Angeles to the fraud investigation, reviewing millions of pages of documents, serving hundreds of subpoenas and interviewing more than 150 witnesses. The internal DOJ code name: "Alchemy," a nod to a medieval practice that included trying to turn common metals into those more noble.