In law enforcement terminology, an “unsub” is an unknown subject of an investigation. This term has been popularized by a number of crime series on television, making many civilians aware of it. Not all law enforcement agencies refer to unknown subjects as unsubs, with usage of the term varying depending on the agency involved and the region in which it is located.
Often, a law enforcement agency will publish information about an unsub in the hopes that members of the general public can identify the subject, making it easier to track him or her down. The agency may distribute a press release which includes photographs of the subject, along with a physical description culled from people who witnessed the crime under investigation or interacted with the subject. By publicizing information, the agency can sometimes receive a tipoff from someone who actually knows the subject, or has seen the subject along his or her travels.
There are a number of reasons for a suspect in a criminal case to remain unknown. While trace evidence can be used to identify many suspects, sometimes it takes a long time to process material gathered from a crime scene, and the agency may want to act quickly to apprehend the subject. In other cases, trace evidence turns up no useful information because the subject is not in criminal databases, or the evidence is compromised in some way.
Law enforcement agencies usually want to act quickly when a crime occurs because the shorter the lead time, the more likely it is that the criminal will be apprehended. By publishing information about an unsub, the agency can hopefully create a form of dragnet, with hotel staff, gas station attendants, and others being aware that they should call law enforcement if they see someone who resembles the unsub.
The other reason for publishing a request for information about an unsub is associated with public relations. Members of a community often become restless or concerned if they cannot see visible action on the part of a law enforcement agency, so by involving the public in the search for the ubsub, the agency shows that it is making progress on the case.
If you are shown a pamphlet or poster about an unsub, read it closely and take note of the identifying features. You should also be aware that some ubsubs are considered dangerous; if a poster tells you that someone is believed to be armed, for example, you should not try to apprehend or stop the ubsub yourself.