The procedures that govern congressional impeachment investigations are largely left to the discretion of the House and, in the case of a trial, to the discretion of the Senate.
A new publication from the Congressional Research Service summarizes the options. It “also describes some of the ways in which an impeachment investigation, as compared to a more traditional investigation for legislative or oversight purposes, might bolster the House’s ability to obtain, either voluntarily or through the courts, information from the executive branch.” Finally, it “briefly describes possible future steps that might follow an impeachment inquiry, including possible action by the Senate.”
See Impeachment Investigations: Law and Process, CRS Legal Sidebar, October 2, 2019
Some other noteworthy new CRS publications include the following.
Ukraine: Background, Conflict with Russia, and U.S. Policy, updated September 19, 2019
Military Space Reform: FY2020 NDAA Legislative Proposals, CRS In Focus, October 2, 2019
American War and Military Operations Casualties: Lists and Statistics, updated September 24, 2019
Defense Primer: Defense Support of Civil Authorities, CRS In Focus, October 2, 2019