But that’s not the same thing as not knowing exactly who were present or involved when things happened, nor is it the same as not obtaining their assistance and testimony in reconstructing what happened.
This person should have been tracked down, 100%. Then having tracked them down, the report should avoid blaming them. It might even be useful to anonymize them after obtaining their testimony, to avoid harassment.
But if we aren’t interviewing them just to avoid blaming them, we have a very, very big meta-problem that needs to be solved.