The detectives often have few or no good clues—they might not even know the victim's identity—and must usually chase several dead ends before finding a likely suspect(s). They start their investigations at the crime scene by talking to any witnesses at the scene while the CSU technicians assist them in the processing of the crime scene as well as determining the proper routing of evidence between the Medical Examiner's office, the Crime Lab and the NYPD Property Clerks office. The CSU has many tools at their disposal to process a crime scene including the materials needed to develop fingerprints, cast footwear and tire impressions, follow the trajectory of bullets fired through windows and the chemicals necessary to observe blood under special lighting conditions that would otherwise be invisible to the naked eye. The unit is also trained to process a crime scene in a hazardous environment, for example following a nuclear, biological or chemical attack.