At the moment, nearly a dozen firms are being examined by the office, according to sources close to the matter, including Goldman Sachs Group Inc., Credit Suisse First Boston, Morgan Stanley, Lehman Bros. Holdings Inc., the UBS Paine-Webber unit of UBS AG, the Salomon Smith Barney unit of Citigroup Inc., Lazard Freres and Bear Stearns Cos. Some of these firms have received subpoenas, while others could get them shortly, the sources said.