In the last few days there have been a few questions using the accident-investigation tag that deal with an ongoing investigation and thus tend to be borderline on topic, if not fully off-topic.
I would propose a pop-up text that would look like the following (simple adjustment of a few words from the accidents pop-up):
Tip: Questions about investigations get better answers if they:
are specific to a part of the investigation, rather than a blanket question (don't just ask "what happened?")
can be answered factually and accurately (speculating about ongoing investigations is explicitly off-topic)
refer to reliable sources like aviation authorities and official accident reports
focus on the aviation aspects of the accident