NEW YORK — Bernard Madoff's contention that he pulled off one of the biggest financial frauds in history without any help is being met with disbelief by his investors and experts in the securities industry.

It normally takes a team of accountants, stock brokers, lawyers and more to operate the kind of multibillion-dollar investment fund that Madoff ran from the 17th floor of his Park Avenue headquarters.

The firm had clients around the globe. Simply generating the detailed financial statements investors got in the mail every month would have been a monumental effort for just one person, observers said, even if those reports were pure fantasy.

"Someone had to create them. Someone had to create the appearance that there were returns," said attorney Harry Susman, who represents several Madoff investors.

"The guy was 70 years old. Could he have done it himself? The computer systems would have needed to be extensive. Supposedly, he's selling puts, buying puts, selling calls, buying stocks. Somebody had to sit there and buy stocks. Where are these people?"

Federal investigators are still in the early stages of trying to answer those same questions as they decipher Madoff's operation. Already, they have discovered multiple sets of books and what appear to be fraudulent documents in his Manhattan offices, raising the question of who prepared them. It may take time before they can say whether he had accomplices.

Investigators have started serving grand jury subpoenas requiring witnesses to testify and have been seeking documents, according to an official familiar with the case. The official, who spoke on condition of anonymity because the investigation is ongoing, declined to identify who was served or specify what documents were wanted.

The investigation has been unfolding in a Manhattan office that was once Madoff's sanctuary but is now the site of a nonstop hunt for incriminating documents, with boxes of confiscated records stacked in the hallway.

The investigation is being led by the FBI and Securities and Exchange Commission — agencies already challenged with unearthing other financial scandals since the Wall Street meltdown.