A Senate panel has subpoenaed financial institutions, including Goldman Sachs Group Inc. and Deutsche Bank AG, seeking evidence of fraud in last year's mortgage-market meltdown, according to people familiar with the situation.

The congressional investigation appears to focus on whether internal communications, such as email, show bankers had private doubts about whether mortgage-related securities they were putting together were as financially sound as their public pronouncements suggested. Collapsing values for many of those securities played a big role in precipitating ...