SnO2 and Mn-doped SnO2 thin films have been grown by radio frequency sputtering in two different atmospheres (Ar and Ar/O-2) on Si(100) and Al2O3 (R-cut) at room temperature (RT) and at 500 degrees C. The RT films are amorphous; those grown at 500 degrees C are polycrystalline or epitaxial, depending on the substrate. All the films, undoped or Mn-doped, present a paramagnetic signal, and ferromagnetism is not observed, regardless of the growth conditions or their structure. The measured magnetization systematically decreases when the films are grown in oxygen-free atmosphere, thus indicating that magnetism is not promoted by oxygen vacancies, and no correlation is found between conductivity and magnetism. We compare the experimental results with ab initio density functional calculations and demonstrate that the Sn vacancies are the origin of the measured magnetization in SnO2 undoped films. Oxygen vacancies contribute to neither the magnetic moment nor the conductivity of the samples. The localized nature of the defect-induced electronic levels prevents a collective magnetic or conducting behavior.