Tower Controller described a loss of separation event when failing to instruct a police helicopter to 'maintain visual separation' with a stadium fly-by flight; the Controller noted the helicopter was at other than his reported location.

Narrative

A law enforcement helicopter departed the airport and flew eastbound 13 minutes prior to event. I immediately asked him if he knew of the impending fly-by at the local stadium at the top of the hour and he stated; that he was going to the east side of the city; the stadium is located on the west side; and that 'he would stay out of their way.' I happened to work this same scenario last year; police helicopter providing security over city and a military fly-by over the stadium with no incident. The subject helicopter actually did not fly to where he stated; but was just on the edge of the stadium. When the fly-by commenced I informed the helicopter of the traffic and he stated he had them in sight. I did not state 'maintain visual separation' as I thought he was at his requested location of 'east of the city;' which would have been sufficient separation. The F-15s did not require any evasive action; but they were preoccupied looking for helicopter traffic that was issued by the RADAR Controller who was working the F-15s. Due to the nature of this event top facility management had a meeting with all controllers and supervisors involved and agreed that the facility as a whole was too lax in such a high profile event; fly-by with F-15s. I certainly fell into this trap as I worked this scenario last year with no incident and 'assumed' all parties knew of each others position; but that was not the case. Recommendation; in our meeting it was decided that we need to sterilize any fly-by area to avoid any potential conflict. All controllers should not assume that all parties are aware of what each other is doing.