NEW YORK - The United States Securities and Exchange Commission has started an informal inquiry into private equity firms, asking for a broad range of documents on how the funds value assets and who invests in them.

The agency's Los Angeles office last year sent letters to several firms asking for details on fund investments and the valuation of assets, as well as communication with clients, according to the copy of a letter obtained by Bloomberg News. Firms were asked to produce the documents by the end of last year.

Private equity firms have come under scrutiny in the aftermath of the financial crisis, which forced firms to mark down holdings acquired during a three-year boom that ended in 2008 when the collapse of Lehman Brothers Holdings Inc froze credit markets. Financial reform measures such as the Dodd-Frank Act have proposed more oversight of the firms' businesses.