High -accuracy coupled cluster methods in combination with microcanonical semiclassical transition state theory are used to investigate a plausible formation mechanism of the simplest Criegee intermediate in a CH4/O-2 discharge experiment. Our results suggest that the Criegee intermediate is produced in a three-step process: (i) production of methyl radical by cleavage of a C-H bond of CH4; (ii) association of methyl radical with molecular oxygen to form a vibrationally excited methyl peroxy, which is in a rapid microequilibrium with the reactants; and finally, (iii) H-abstraction of CH3OO by O-2, which results in the formation of cool CH2OO, which has insufficient internal energy to rearrange to dioxirane.