 Local procedures required the east controller to wait until the departure on 15R had passed through the intersection before clearing the aircraft on runway 9 for takeoff.The NTSB reported that the probable cause of the incident was that the east local controller failed to follow FAA Order 7110.65 and local procedures which resulted in a runway incursion.After the incident, the Boston tower changed its procedures so that only the west local controller may initiate a departure on the crossing runway 15R, and that once the east controllers accepts the release the aircraft must be cleared for takeoff within five seconds.