  Using a three-dimensional quasiclassical technique we explore molecular
double ionization by a linearly polarized, infrared (800 nm) and ultrashort (6
fs) laser pulse. We first focus on intensities corresponding to the tunneling
regime and identify the main ionization mechanisms in this regime. We devise a
selection of observables, such as, the correlated momenta and the sum of the
momenta parallel to the laser field as a function of the inter-electronic angle
of escape where all the main mechanisms have distinct traces. Secondly, we
address intensities above but close to the over-the-barrier intensity regime.
We find a surprising anti-correlation of electron momenta similar to the
experimental observations reported in Phys. Rev. Lett. \textbf{101}, 053001
(2008). There, however, the anti-correlation was observed in very low
intensities corresponding to the multiphoton regime. We discuss the mechanism
responsible for the anti-parallel two-electron escape.
