The burden of proof is very high. The U.S. government is the ultimate arbiter of who is put on that list and who isn't. A person put on the list has the opportunity to challenge it in federal court. The government will only put a person on that list when they can defend doing so to a criminal standard based on evidence, so it is not according to a civil but to a criminal standard that they are placed on that list.
On April 29th, 2014. See this statement in context.