WASHINGTON -- The Securities and Exchange Commission is examining the books of major Wall Street banks to determine their vulnerability to home-loan defaults, a person familiar with the investigation said.

The person, who declined to be identified by name because the inquiry has not been publicly disclosed, described the examination as "Street-wide" and a routine part of the SEC's oversight authority.

The inquiry was reported Friday by The Wall Street Journal, which said Goldman Sachs Group Inc. and Merrill Lynch & Co are among the first companies to be examined.

SEC Chairman Christopher Cox disclosed in late June that the SEC had started about a dozen investigations related to complex aggregations of mortgage debt known as collateralized debt obligations, which investors around the world purchased in recent years.

John Nester, an SEC spokesman, declined to confirm or deny the SEC's activity, other than to say that how to value mortgage securities "is always a concern" because the investments trade infrequently and are difficult to price.

"Valuations are something that our inspection teams are absolutely focusing on when they do inspections," Nester said.