The US Supreme Court has ruled that The National Labor Relation Act gives workers the right to request union representation during investigatory interviews by supervisors, security personnel, and other managerial staff. These are called Weingarten Rights.
An investigatory interview occurs if 1) management questions you to obtain information: and 2) you have a reasonable apprehension that your answers could be used as a basis for discipline or other adverse action.
You must ask for union representation either at the beginning of or during the interview. Management does not have to remind you of this right.
If your request is refused and management continues asking questions, you may refuse to answer. Your employer is guilty of an unfair labor practice and charges may be filed.