The Securities and Exchange Commission said Tuesday night that it had missed repeated opportunities to discover what may be the largest financial fraud in history, a Ponzi scheme whose losses could run as high as $50 billion.

The commission said it received credible allegations about the scheme at least nine years ago and will immediately open an internal investigation to examine why it had failed to pursue them aggressively.

The S.E.C. issued the statement hours after Bernard L. Madoff, the 70-year-old Wall Street executive accused of operating the scheme, discussed the fraud with federal authorities at a meeting in New York on Tuesday, according to people briefed on the meeting.

“Our initial findings have been deeply troubling,” Christopher Cox, the S.E.C. chairman, said in his statement. The commission received “credible and specific allegations regarding Mr. Madoff’s financial wrongdoing,” but did not respond aggressively, he said.