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The collective motion of bacteria leads to intermittent jets and swarming vortices, a fluid pattern often referred as bacterial turbulence. We investigate the emergence of the collective motion of Escherichia coli suspensions and explore the kinetic pathway towards bacterial turbulence. We map the phase diagram of bacterial flows as functions of bacterial concentration, bacterial swimming speed and the number fraction of active bacteria. A simple model based on two-body hydrodynamic interaction quantitatively predicts the phase boundary of the 3D phase diagram. Furthermore, we trigger bacterial turbulence by using genetically engineered light-powered E. coli, whose swimming speeds vary with light intensity. The kinetics show one step near the phase boundary and two steps with an intermediate state far above the phase boundary. The transition rate increases as the system moves deep inside the turbulent phase. Our research reveals the microscopic origin of bacterial turbulence and provides new insights into nonequilibrium phase transitions in active matter.