42 CFR § 93.518 - Witnesses.
(b) The ALJ may admit written testimony if the witness is available for cross-examination, including prior sworn testimony of witnesses that has been subject to cross-examination. These written statements must be provided to all other parties under § 93.513.
(c) The parties may conduct direct witness examination and cross-examination in person, by telephone, or by audio-visual communication as permitted by the ALJ. However, a respondent must always appear in-person to present testimony and for cross-examination.
(d) The ALJ may exercise reasonable control over the mode and order of questioning witnesses and presenting evidence to -
(1) Make the witness questioning and presentation relevant to deciding the truth of the matter; and
(2) Avoid undue repetition or needless consumption of time.
(e) The ALJ must permit the parties to conduct cross-examination of witnesses.
(f) Upon request of a party, the ALJ may exclude a witness from the hearing before the witness' own testimony. However, the ALJ may not exclude -
(1) A party or party representative;
(2) Persons whose presence is shown by a party to be essential to the presentation of its case; or
(3) Expert witnesses.