In almost all cases, yes. You need to establish the facts before taking any formal action or holding a hearing. The only real exception is where the facts genuinely are not in dispute.
Skipping the investigation, or rushing it because the conduct seems obvious, is one of the most common reasons employers lose tribunal cases they should have won.
Important
Every situation has its own details, and those details change the right answer. For a read on yours, take the free Employee Situation Check or book a call.
General guidance to help you decide your next step, not legal advice. Employment law changes, so always check the current position for your own situation.