Department of Homeland Security: Review of Report on Agency Estimates of Foreign Nationals Unlawfully Residing in the U.S., Letter to Congressional Committees [open pdf - 758KB]
This is a September 10, 2019 letter and draft report regarding a Department of Homeland Security Review, from Rebecca Gambler, Director of Homeland Security and Justice. From the Letter: "Under U.S. immigration law, there are various types of immigration benefits that provide lawful immigration status or presence in the United States on a temporary or permanent basis. Foreign nationals in the United States without valid immigration status or protection are unlawfully present. Information about the size and composition of the unlawfully present population can help inform immigration and border security-related policy decisions. The Department of Homeland Security's (DHS) Office of Immigration Statistics (OIS) has the primary responsibility for collecting and disseminating statistical information and analysis used in evaluating the impacts of immigration laws, migration flows, and immigration enforcement."
Government Accountability Office: http://www.gao.gov/