What do I do when there is no one to interview during a workplace investigation?

What do you do if you have an investigation but there is no one to interview? This can come up because there was nobody around or because our witness made themselves unavailable. Those are really hard. So acknowledge to yourself that you got a doozy of a case and then get to work meeting your 'more likely than not' standard. We can interview other people who engage with our accused often and say, is this the kind of thing they do? Would you be surprised if they did this thing? Of all the people here, who do you think might have done this thing? There are some other questions that we can ask to kind of get around to that more likely or not standard, even if there's nobody directly to an interview on the topic.