Dealing with suspects of sabotaging experiments In a group, postdoc A believes postdoc B sabotaged her experiments. A shares her frustration with the PI (without proof) in a "half-jokingly, half-serious" sense. Since then, strange things appear to be happening in the lab -- specifically, B's experiments are behaving strangely. There has been a serious decline in trust between A and B. 

How should the two postdocs proceed? Should they meet? Should the PI mediate between the two? Should the PI talk to them separately? Or together? 

Ultimately, how should the PI address the problem, knowing that an investigation is in practice impossible and with the goal of reestablishing a minimum level of mutual trust?

EDIT: Pushed by the PI, the 3 actors openly talked about these issues, agreeing that sabotage is extremely serious, but without pointing the finger to anyone. That seems to have released quite a lot of pressure and helped to normalized the situation. Hopefully it will last.
That would be the time for the PI to call a meeting with A and B and have a conversation among adults. Topics to be discussed are: If A has a grief with B, she needs to be specific and put it on the table. If B suspects A, he needs to do the same. PI needs to say that this is unacceptable and that everyone needs to behave like adults or else.

If the PI does not call this meeting, then either A or B need to be the grown-up in the room and do so.

Ultimately, half-truths, allegations, conspiracy theories, "experiments not working well", etc, is a situation that is to nobody's benefit. The only way to address it is to talk about it.