A police officer investigating a crime or a crime scene is also responsible for gathering witness statements, or locating witnesses, but finding and interviewing them is not enough.  Considering the credibility of the witness and their account of events is critical in investigations.

Any number of internal factors can influence their credibility.  For example, is the witness a member of the City Council or a known drug user?  One statement would be considered more reliable and would hold up better in court.  The motive of the witness is also key.  Are they a relative of the victim?  Are they a member of a rival gang?

The mental and physical state of the witness is also important.  Were they sober when they "saw" what happened? Are they sober now? Is the person mentally ill, or simply tired? 

Experienced law enforcement officers are trained to spot conflicting motivations and conditions which might throw off an investigation or lead to the discounting of the witness statement.