By Quinn Dombrowski from Berkeley, USA (Nancy Pelosi Uploaded by Anastasiarasputin) [CC BY-SA 2.0 (https://creativecommons.org/licenses/by-sa/2.0)], via Wikimedia Commons
As of December 2019, the Democrats have completed the investigation aspect of impeachment, despite many claiming that this was poorly done. The fact of the matter is that the Constitution does not require an investigation approved by all parties, it is the parties in power alone that get to pick the way these investigations are run.
The Democrats did break with the traditional impeachment investigation pattern by having several committees run investigations as opposed to leaving it up the Judiciary Committee.
Source: TTN staff