The danger is real, as demonstrated by the horrible murderous terrorist attacks on US soil, including the most recent one on the National Guard soldiers this Thanksgiving. The US gov't has legal grounds to stop it - now.
US President Trump’s Executive Order (EO), initiating the process whereby various subdivisions of the Muslim Brotherhood (MB) are each to be designated as a Foreign Terrorist Organization (FTO), as well as for specially designating persons as global terrorists, is indicative of the complexity of this undertaking.
The MB is not organized like a US public company entity, with a formal corporate structure, including legal subsidiaries and affiliates, as disclosed in public filings and subject to regulation.
Instead, the MB ostensibly is loosely organized to mask its malign activities and frustrate the taking of appropriate criminal and civil enforcement action under US law, including seizing assets. Activists, with a common purpose and some form of common funding, are bound together through coordination and collaboration, often by means of players associated with a Foreign Terrorist Organization (FTO), like Hamas, PFLP or Hezbollah, or otherwise supporting any of them or their nefarious agendas.