Before you can open fire on another gangster you must locate them using detectives. The higher your target level, the harder it will be to find them.
You can locate a gangster from any level. To locate them you must choose what locations you wish to search, how long the detectives should spend searching and how many detectives should be in each location.
The higher your target the longer you should have your detectives to search for and the more detectives you should use. You can't hire more than 100 detectives per location at a time.
By default your detectives will track a target for 6 hours if they find them and if they travel away once found then you no longer have them tracked.
You can pay extra to have your detectives follow the target once found. This means that if your detectives find your target but the target travels to a new location, the detectives will continue tracking them in the new location. However, if your target was to travel for a second time then your detectives will not continue tracking them. This is very useful if you are trying to search for an active player who could travel as you prepare to shoot.
There is no guarantee you will find a target. However, you can increase your chances by hiring multiple sets of detectives for your target.
The price of your detectives depends on how long they will be searching, where they will search, how many will search in each location and whether they will follow the target.