More than 30 Maine doctors have reported to the Maine Medical Association that fraudulent tax returns using their names and Social Security numbers were filed this year. The cases could be linked to a broader breach of doctors' personal information that the U.S. Secret Service is investigating. Gordon Smith, executive vice president of the Maine Medical Association, said in a phone interview Thursday that approximately 35 physicians have notified him of fraudulent tax returns that were filed using their personal information. George Ogilvie, spokesman for the Secret Service, which investigates cases of credit card fraud and identity theft, confirmed in a phone interview Thursday that the agency is looking into the matter.