plaster

Can't seem to find any information on this.

Scenerio:

Unsolved murder in uppity suburb town by detectives that really have no idea what they are doing.

Police initial investigation:

Is there an unwritten law of how police would investigate for a possible suspect? Specifically, if they had no idea who would commit this crime, would they do a background check for felons in the local area with similar violent crimes? And if there is some type of normal procedure to do this background check, is there a common number of years they would check back on? If so, how many years would you think (or know with some real life experience) would be the norm?

Just curious.