Occasionally, the official first contact takes place years or even decades after members of the species involved have first met. Typically, first contact is only initiated by the Federation if a civilization is sufficiently advanced and has developed interstellar travel (via warp drive, for example). Contact with more primitive civilizations is prohibited by the Prime Directive. It has been stated that the preferred method is to privately approach scientists and intellectual leaders to make the initial contact, on the assumption that they would more easily grasp the concept of alien life. Also, the population is typically observed covertly for some time before making first contact, a decision made after the Klingon first contact.