In certain circumstances, yes, I find it possible.
You don't have to be a witness to the event to offer specialist knowledge, if you have any: e.g., engineers, builders, architects contacting the police to alert them to details of a particular construction / demolition in the area of interest, for example.
Or, if someone suddenly stumbles on something odd while clearing out a deceased great-uncle's attic that raises a suspicion (still not a direct witness to the events).
Or, any member of the public who notices something potentially relevant that the police may not have been aware of even if it turns out to be a dead-end.
However, this doesn't appear to be the case in this latest tabloid saga.
Perhaps I should have made my definitions clearer but should I do so I suspect one of Cerberus' heads will delete it for being non PC.
I wasn't thinking of the expert who knows what he is on about spotting something that is of use.
I was thinking more of the professional interfering busybody who fancies their chances as Jane Marple or Hercule Poirot.
How many of them make a significant contribution ?