From the Summary: "The Federal Bureau of Investigation (FBI, the Bureau) has acknowledged that it had been investigating the shooter who killed 49 people at an Orlando nightclub on June 12, 2016. The gunman has been identified as Omar Mateen, a 29-year-old security guard in Florida who was born in New York. Reportedly, Mateen was watchlisted while under FBI investigation. This report provides background information on the watchlisting process. The Terrorist Screening Database (TSDB, commonly referred to as the Terrorist Watchlist) lies at the heart of federal efforts to identify and share information about identified people who may pose terrorism-related threats to the United States. It is managed by the Terrorist Screening Center (TSC) and includes biographic identifiers for those known to have or those suspected of having ties to terrorism. It stores hundreds of thousands of unique identities. Portions of the TSDB are exported to data systems in federal agencies that perform screening activities such as background checks, reviewing the records of passport and visa applicants, and official encounters with travelers at U.S. border crossings."
CRS Report for Congress, R44529
Congressional Research Service: https://crsreports.congress.gov/