Confirmation bias
Confirmation bias is a psychological phenomenon whereby people tend to seek and interpret information in ways that support existing beliefs. Two mechanisms come into play - selectively searching for supporting information and a biased interpretation of whatever information is available. In police investigations, this comes into play when detectives identify a suspect early in an investigation, come to believe he or she is guilty, and then ignore or downplay other evidence that points to someone else or doesn't fit their hypothesis about what occurred.