U.S. regulators have opened a new line of inquiry in their mortgage foreclosure probe and are asking big Wall Street banks about the beginning stages of mortgage securitization, two sources familiar with the probe said. The Securities and Exchange Commission launched the new phase of its investigation by sending out a fresh round of subpoenas last week to big banks like Bank of America Corp, Citigroup Inc, JPMorgan Chase & Co, Goldman Sachs Group Inc and Wells Fargo & Co, the sources said. The SEC’s subpoenas focus on the earliest stage of the mortgage securitization process, said the sources, who requested anonymity because the probe is not public. The sources said the SEC is asking for information about the role of so-called “master servicers” — specialized firms that oversee the selection and maintenance of the large pool of home loans that go into every mortgage-backed bond.