She joined the Superior Centre for Defense Information (CESID) in 1983 when General Emilio Alonso Manglano was director. Esteban specialized in foreign intelligence, although she has never worked as a field agent. Her first jobs within the intelligence service were the preparation of reports on the permanence of Spain in NATO, before the 1986 referendum. She also elaborated, as an analyst, reports relating the 11-S and 11-M terrorist attacks.