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Gas is one of the promising and convenient medium for generation of broadband pulsed terahertz radiation. The highest THz generation efficiency is reached in the case of high intensity dual-frequency femtosecond laser pulses. The most important contribution to generation of low-frequency field is provided by the photocurrent of free charges, induced by photoionization, and the four-wave mixing process, which describes nonlinear response of bound electrons in a gas. In this paper we show both theoretically and experimentally that the THz-range radiation produced by co-propagating in nitrogen and air 800 nm and 400 nm high-intensity femtosecond laser pulses consists of the contribution from both the free electron current and the nonlinear polarization of neutral molecules. The major contribution from free electrons is in the low-frequency part of the THz spectrum, while the higher-frequency part is dominated by the instantaneous and delayed Kerr nonlinear response of neutrals at the difference frequency. We discussed the polarization properties of the THz radiation versus the polarizations of the incoming light beams.