The unusual nature of how Bernard Madoff's alleged $50 billion Ponzi scheme surfaced -- with a confession from the key figure himself -- is complicating a high-profile investigation by the Securities and Exchange Commission. ..... Typically, investigators begin with a tip or whistleblower and cut deals with lower-ranking employees to build a case against the suspected mastermind of a fraud. In this case, investigators started with a confession from the mastermind, Mr. Madoff, according to a government affidavit, and they are trying to work backward to piece together what happened. People familiar with the investigation say the Madoff firm's records...