Commissioning with cosmic rays of the Muon Spectrometer of the ATLAS experiment at the Large Hadron Collider

The Muon Spectrometer of the ATLAS experiment is made of large toroidal magnets, arrays of high-pressure drift tubes for precise tracking and dedicated fast detectors for the first-level trigger. All the detectors in the barrel toroid have been installed and commissioning has started with cosmic rays. These detectors are arranged in three concentric rings and the total area is about 6000 square meters. During the installation and commissioning of the detectors, data are usually taken with the magnet off, but a dedicated run took place with the magnetic field of the barrel toroid turned on. We present the procedure to control the response of the single detectors installed in the barrel toroid, Monitored Drift Tubes and Resistive Plate Chambers, and results of the first tests done with cosmic rays triggered by the first-level processor and read-out trough the ATLAS data acquisition. Details on the installation and commissioning schedule will be given in view of the completion of the instrumentation of the muon spectrometer for the first period of data taking with proton-proton collisions.