Before formal resolution by the House, congressional committees may subpoena witnesses & investigate. E.g., Watergate hearings were May-June 1973, the House Judiciary Committee authorized an investigation in Oct. The full House voted to initiate impeachment proceedings Feb 1974.https://twitter.com/annabella313/status/1140065947807571970 …
Yes and the house is holding hearings and subpoenaing witnesses now. However, until they win the subpoena wars in court, very few witnesses are willing to show up. A hearing without a witness is useless.