Investigation of nucleation during solidification on an atomic scale is challenging with both experimental and theoretical methods due to the involved time and length scales. Since nucleation time scales are many orders of magnitude greater than that achieved by conventional molecular dynamics, we employ transition path sampling, an atomistic simulation method that generates an ensemble of trajectories between the liquid and the solid states. From the path ensemble, thermodynamic and kinetic properties such as the free energy barrier and the nucleation rate can be calculated. Furthermore, it also allows the analysis of the nucleation mechanisms. We study homogeneous nucleation in Mo at various undercoolings and find that the crystal core originates in a region of pre-ordered liquid, with BCC being the most abundant phase throughout nucleation. We also compare the mechanism to that of nickel to reveal that the selection of the final structure happens in the early stages of nucleation.