The phenomenon of ultra-fast gas breakdown (nanosecond time scale) attracts large attention due to the interesting physical processes and important practical applications. The research was carried out in pressure range of 1-7 atm., in air, using nanosecond high-voltage (HV) all-solid state generator for initiation and maintenance of air discharge inside the cathode-anode gap. The temporal and spatial parameters of the discharge were studied using optical fast-framing imaging, x-ray time resolved diagnostics, electron beam imaging and energy spectrum diagnostics.  The use of the temporally-resolved X-ray and light diagnostics provided an insight to an origin of runaway electrons in the discharge.