Congressional Access to Information in an Impeachment Investigation [October 25, 2019] [open pdf - 1MB]
From the Introduction: "Committee investigations in the House of Representatives can serve several objectives. Most often, an investigation seeks to gather information either to review past legislation or develop future legislation, or to enable a committee to conduct oversight of another branch of government. These inquiries may be called legislative investigations because their legal authority derives implicitly from the House's general legislative power. Much more rarely, House committee investigations have been carried out to determine whether there are grounds to impeach a federal official--a form of inquiry known as an impeachment investigation. An impeachment investigation has typically been one of the House's first steps in the exercise of its constitutional impeachment power, and may conclude with the investigating committee recommending articles of impeachment to the full House."
CRS Report for Congress, R45983
Congressional Research Service: https://crsreports.congress.gov/