Congressional material (committee hearings and reports, etc.) can be used both as primary source material, for example as a means of examining political discourse, or as background research information.
Executive branch material consists of the Executive Office of the President and the vast regulatory apparatus that is charged with carrying out the laws passed by Congress. The primary government office charged with law enforcement and migration is the US Immigration and Customs Enforcement. The portal for government information on immigration is WelcometoUSA.gov.
The Federal Register is published daily and consists of proposed and passed regulations that Federal agencies use to carry out policies based on laws passed by Congress. The Code of Federal Regulations is the yearly compilation of promulgated rules and is organized into 50 titles. Federal regulations exist to cover almost every aspect of migration and immigration, including such areas as employment, sex trafficking, education, and residence eligibility.