Investigators from the Securities and Exchange Commission rushing to uncover Bernard Madoff's alleged fraud last week found records from what appear to be thousands of clients but ran into trouble with a key Madoff lieutenant who was evasive, according to an SEC memorandum reviewed by The Wall Street Journal.

The SEC and other investigators are also examining the activities of Mr. Madoff's wife, Ruth, and what role she may have played in the alleged $50 billion Ponzi scheme that is believed to have lasted two decades, people...