McClane finds the explosives and realizes what Gruber is up to .
When he finds Gruber working at the explosives planted on the roof , Gruber passes himself off as a terrified hostage who escaped . McClane makes friendly conversation and gives Gruber a pistol ; when Gruber attempts to fire the weapon at McClane , however , it is discovered that McClane inserted an empty magazine in the gun and gave him the gun to test his trustworthiness .