NDA's do not apply in criminal investigations,period, and not even in some civil lawsuits.
According to my sister, who is a practicing attorney, the main thing NDA's do is give you an justification for firing somebody and terminating their contract if they break them. Other then that, they are fairly useless.
Their usefulness does depend a lot on what state you are in. They would not be at all useful for an employee of the White House, in any legal sense. This is simply an intimidation tool.
