One of the barriers to effective criminal investigation is the unwillingness of potential witnesses to a crime to come forward or to talk to investigators. It is very common for people not to want to get involved, because they are worried for their own safety or that of their family. In this instance, the detectives can overcome that barrier by blanketing a neighborhood and talking to anyone who lives there. Odds are, they will find someone who is an actual witness who is willing to talk to them.
Another barrier is the integrity of the crime scene. When physical evidence is destroyed or compromised, it greatly complicates the task for both investigators and prosecutors of identifying suspects and building a case. By observing proper police procedure, cordoning off the crime scene and properly gathering, marking and storing evidence, police can often overcome this barrier too.