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This paper deals with large-scale gravitational-wave detector control. This detector is a laser illuminated large-scale interferometer which uses multiple optical elements and senses displacements as small as 10 -18m produced by gravitational waves. To make the detector operant, the variations of the cavities lengths must be sensed and corrected. The optical cavities are composed by mirrors suspended on vibration-isolated weighted pendulums which are used in order to attenuate the seismic noise due to the Earth motion. However the mirrors' residual displacements once suspended to the damping system must be reduced by an order of 106 to make the detector operant. In this paper a linear quadratic Gaussian control law with loop transfer recovery (LQG/LTR) is designed in order to meet the requirements on seismic noise attenuation.