Synonyms and related words :
bugging, chase, chasing, cloak-and-dagger work, consecutive, counterespionage, counterintelligence, dogging, electronic surveillance, espial, espionage, follow, follow-up, following, heeling, hounding, hue and cry, hunting, intelligence, intelligence work, military intelligence, observation, prosecution, pursual, pursuance, pursuing, pursuit, quest, searching, secret police, secret service, seeking, sequel, sequence, series, shadowing, spying, stakeout, stalking, succeeding, surveillance, tailing, tracking, tracking down, wiretap, wiretapping