The results of a series of two-dimensional numerical particle simulations and a linear analysis of the collisionless tearing mode instability are presented. Comparisons are made between codes which are run with (1) ion dynamics only and magnetic field confined to the plane of the simulation.(2) ion and electron dynamics, including electrostatic interaction, and magnetic field in the plane of the simulation, and (3) ions and electrons and all three components of the magnetic field, as well as the electrostatic field. It is found that the addition of electrons significantly reduces the linear growth and decreases the wavelength of the most linearly unstable mode. The growth rate and wavelength of the most linearly unstable mode are found to be consistent with linear analysis. The addition of the magnetic field component perpendicular to the plane of the simulation has a negligible effect on the linear growth rates or on subsequent nonlinear behavior. Significant electrostatic fields are generated in the nonlinear explosive phase. Somewhat larger potentials are generated in run 3 because of the finite component of the magnetic field normal to the plane of the simulation.