The following sources are directories of government agencies and will provide a list of key personnel in each agency, including their position and contact information (which may be historical):
A directory of government agencies in a pdf format. Just choose a year and browse for the specific agency. It will list the individuals and their roles, down to the manager level in most cases. Coverage in 1998 through 2010.
Federal Yellowbook (Leadership Library on the Internet)
A searchable database of government directory information for the current year. It lists employees down to the assistant level and includes past affiliations for current employees.
U.S. Government Manual
Includes information on government agencies in a pdf format. Just choose a year and browse for the specific agency. It will list the administrative individuals as well a providing and overview of the agency itself. Coverage in 1998 through 2010.