It is shown for the first time that Co subnanometer coatings deposited by molecular-beam epitaxy on the (0001) surface of the topological insulator BiSbTeSe(2)at a temperature of 330 degrees C open an energy gap in the spectrum of topological surface states in the region of the Dirac point with a shift of the Dirac-point position caused by the preliminary deposition of an adsorbate at room temperature. The gap width is 21 +/- 6 meV. Temperature-dependent measurements in the range of 15-150 K show no changes in the energy-gap width.