28 U.S. Code § 2246 - Evidence; depositions; affidavits
On application for a writ of habeas corpus, evidence may be taken orally or by deposition, or, in the discretion of the judge, by affidavit. If affidavits are admitted any party shall have the right to propound written interrogatories to the affiants, or to file answering affidavits.
This section is derived from H.R. 4232 introduced in the first session of the Seventy-ninth Congress by Chairman Sumners of the House Committee on the Judiciary. It clarifies existing practice without substantial change.