45 C.F.R. § 1174.29 – Witness testimony.

(a) Except as provided in paragraph (b) of this section, testimony at the hearing shall be given orally by witnesses under oath or affirmation.
(b) At the ALJ's discretion, the ALJ may admit testimony in the form of a written statement or deposition. The party offering such a statement must provide it to all other parties along with the last known address of the witness, in a manner which allows sufficient time for other parties to subpoena the witness for cross-examination at the hearing. The parties shall exchange deposition transcripts and prior written statements of witnesses proposed to testify at the hearing as provided in § 1174.22.
(c) The ALJ shall exercise reasonable control over the mode and order of interrogating witnesses and presenting evidence.
(d) The ALJ shall permit the parties to conduct such cross-examination as may be required for a full and true disclosure of the facts.
(e) Upon any party's motion, the ALJ shall order witnesses excluded from the hearing room so that they cannot hear the testimony of other witnesses. This rule does not authorize exclusion of—
1. (1) A party who is an individual;
2. (2) In the case of a party that is not an individual, the party's officer or employee appearing for the entity pro se or designated by the party's representative; or
3. (3) An individual whose presence a party shows to be essential to the presentation of its case, including an individual employed by the Government or engaged in assisting the Government's representative.