The service relocated to Blenheim Palace for much of the Second World War and then moved to Leconfield House after the war before relocating to 140 Gower Street in 1976. The dispersed and dilapidated state of its previous buildings at 140 Gower Street (headquarters) and Curzon Street House (registry, administration and technical services) led MI5 to seek a new home in the late 1980s. The Secret Intelligence Service (MI6) were engaged in a simultaneous hunt for new headquarters and consideration was given to co-location of the two. However this proposal was abandoned, due to the lack of buildings of adequate size (existing or proposed) and the security considerations of becoming a single target for attacks. At the same time, Thames House, which was largely used as government offices by then, became vacant when the Department of Energy left the southern half in 1989 and it was decided to convert and refit much of it for MI5's use. The GMW Partnership undertook the design and Mowlem carried out the necessary reconstruction work from 1990, which included part-infilling of the building's distinctive archway. An automated miniature monorail within the building brings files up from the basement of Thames House to staff working within.