An investigation by the Senate Homeland Security Committee alleges that the FBI, the Department of Homeland Security and leading social media companies are not adequately addressing the growing threat of domestic terrorism, especially white supremacist and anti-government extremists. In a 128-page report obtained by NBC News, the committee’s majority Democrats say federal law enforcement agencies have not appropriately allocated resources to match the metastasizing threat, and have failed to systematically track and report data on domestic terrorism incidents, as required by federal law. “Unfortunately, our counterterrorism agencies have not effectively tracked the data that you need to measure this threat,”...