The Department of Homeland Security (DHS) has a website with information on a number of different topics. This is the National Threat Assessment Center. It is described below:
“The National Threat Assessment Center (NTAC) was established as a component of the Secret Service in 1998 to provide research and guidance in direct support of the Secret Service protective mission, and to others with public safety responsibilities.
“The center’s staff is composed of a multidisciplinary team of social science researchers and regional program managers who support and empower our partners in law enforcement, schools, government, and other public and private sector organizations to combat the ever-evolving threat of targeted violence impacting communities across the United States.”
They have just released at report, Mass Attacks in Public Spaces: 2016-2020. Go to the website linked above and it is the first item on the left — a PDF of the report.