The Homeland Threat Response Act amends the Homeland Security Act of 2002 to broaden the scope of federal assistance available for violent incidents. It specifically authorizes U.S. Customs and Border Protection (CBP) to deploy and provide support in relation to certain violent acts, shootings, and mass killings. This legislation modifies section 875(d)(1) of the Act, adding CBP to the list of agencies, alongside U.S. Immigration and Customs Enforcement, that can offer assistance. The bill also expands the types of support provided to include response, threat mitigation, and resolution , in addition to investigation, for these critical events.