Consider the children's game Chinese Whispers. Players line up such that they can whisper into the ear of their immediate neighbours. The player at the beginning of the line thinks up a phrase and whispers it to the next player. This player in turn passes on the received message to the next player, and so on. The last player in the line calls out the message she received.
Each information transfer between two children may carry changes to the phrase, i.e. a loss, a change, or an addition to the incoming message. Only a fraction of the incoming information content survives each information transfer. The information content of a message after n transfers I(n) is a function of the fraction information content that survives each transfer s(i), where I(0) is the initial information.
I(n) = s(1) * s(2) * s(3) * ... * s(n) * I(0)
The final, called-out phrase may bear little resemblance to the initial phrase. Reasons for the changes to the phrase include difficulty in uttering and understanding whispers, anxiousness, impatience, erroneous corrections, or deliberate alterations.
It is these losses, changes, and additions that you want to avoid in your investigation. That is why you should aim for the primary source of information.
Most information you assimilate has a long information trail:
1: A citizen files a complaint about police brutality.
2: The Internal Affairs Department starts an investigation.
3: The investigation report is sent to the Public Relations Department.
4: A Police spokeswoman holds a press conference.
5: A journalist writes up an article on police brutality.
6: A co-worker reads the article.
7: You listen to the co-worker talking about police brutality.
Seven steps, six information transfers.
Even if you cannot get to the primary source of the information, you should aim to come as close as possible.