After these inconclusive attacks, the Ottomans sought to break through the walls by constructing tunnels to mine them from mid-May to 25 May. Many of the sappers were miners of Serbian origin sent from Novo Brdo under the command of Zagan Pasha.^[15] An engineer named Johannes Grant, a German who came with the Genoese contingent, had counter-mines dug, allowing Byzantine troops to enter the mines and kill the miners. The Byzantines intercepted the first tunnel on the night of 16 May. Subsequent tunnels were interrupted on 21, 23 and 25 May, and destroyed with Greek fire and vigorous combat. On 23 May, the Byzantines captured and tortured two Turkish officers, who revealed the location of all the Turkish tunnels, which were destroyed.^[16]