Well can they? I've been approached on a number of occasions by those affected by another's crime against someone they love or when someone has uncharacteristically gone missing.
From my point of view it's much easier to work with the energy apparent before other mediums, dabblers and those with opinions. The fresher the energy the better for me.
The problem when others start poking about is that the energy becomes polluted and confusion sets in for anyone trying to reach the truth. Other problems are when the medium picks up on the wishes or beliefs of the person asking for help. For example if a parent is missing a child - no matter what their age - their hope will be that the child is alive and will be found. The medium then working with the energy has to decide between what they receive perhaps coming from the parent or the energy surrounding the victim/missing person. And that is not easy. Nor is working with distraught relatives who look to you to make their dream come true.
Even working through guides the medium's own thoughts will intrude and their own experiences and hopes can also influence what they receive. A good medium will be able to sift through the information objectively but it is a very hard thing to do when the desire to help is too strong. All information needs thinking about and collating carefully before passing to the relatives.
Another point to mention is that a medium or dabbler should only ever give information when invited to do so. It is a complete invasion of privacy to pass on details received to others without being asked for them and can cause more harm than good. (I add to this the same advice for giving healing uninvited and giving readings without being asked or telling anyone that they have spirit with them without their request. Do not do it!)
Sometimes it can seem very clear that you have something to give. You have had a vision or a vivid dream or have worked the cards (tarot perhaps) to glean knowledge about a case near to you or in the news. But beware, there are those spirits that would wish to distract you from the good work you should be doing and also to confuse the energies surrounding the case you wish to work with. Not all spirits have good at heart and not all interesting cases need your input.
We have to take into account the existence of Karma with all things too. We can never look at events with just the physical eye, we need to address issues with a spiritual view point too and from different points of perspective. No situation is formed by one reason, one thought or just one action, many lives cross, many thoughts gather and many actions build to lead to the outcome.
Say for example that a man kills a woman. The statement is gruesome but simple but the facts do not end there. There is a complication to consider when examining the energy evidence and that is Karma. There will be reasons unknown to those not within the soul group of those affected by the killing. There will be an agreement and understanding of the souls of the victim and perpetrator but their physical selves will not remember that and nor will the members of their soul group. The offence will create anger and grief which adds to the energy pool so the task of the medium is to pick through valid senses and ignore those that don't feel genuine and could be the work of her own imagination and fears or that of others. A difficult job.
So what can the medium do to help the process? She/he must make sure their own mind is clear of negativity, clear of hopes or fears for the case, indifferent even to the outcome. Be clear about intention when working with the energy and not manipulate it to suit her own agenda. Make strong connections with guides and spirits helping with the investigation. By that I mean making the connection so strong that no interference can intrude, that communication with guides and spirit is exclusive. Question everything. Phrase questions carefully. Spend a limited amount of time when working so as not to become absorbed into the case too deeply, detachment is key.
So can a medium help? There are very few cases where they have genuinely helped to locate a body or solve a crime. Often the authorities don't like to admit to the fact that a medium has been used but it does happen. What seems to happen most often is that the medium will give some clues, locations, details etc., but nothing solid and the events will play out in time revealing details and truths as they go. If for instance it is the parents' Karma to lose a child and live their life in grief, then that is what will happen no matter who tries to solve the case. The universe works in ways that we don't always understand but our spirit and soul always have understanding and will find peace in the end.
When we are in our physical body and think in physical terms it is difficult to accept spiritual laws and occurrences that give us grief and pain and even though I work as a medium, I am no exception to that. The physical body enables us to feel these things that the spirit self cannot and it is invaluable to the soul.
I see my role as a medium when helping people with loss of some kind to educate them about the spirit world and how the laws of spirit work to bring them comfort and acceptance of their situation. If I do make contact with the spirit of their loved one I will give honest information and only what the spirit bring, nothing of my own will be added because it wouldn't be truthful. An open heart is all that's needed and a desire to bring peace to others. The problem with many who try to help is that they want some kind of glory from it and that involves the ego, no good ever came from that.
There are those mediums that seem to specialise or be chosen to work in that field and have a measure of success but often such cases are left open ended and are difficult to solve by traditional means. The medium can be brought in through desperation and as a last resort rather than when the energy trail is fresh and of more use to the sensitive empath. Things are always as they should be no matter how horrible or painful and we must hold on to the fact that we are growing and learning all the time and everything we experience in the physical body is desired by the soul.