The mechanisms and the sequence of crystallization of a Fe64B24Y4Nb6Al0.4 metallic glass were investigated experimentally. To this end, the microstructure of the metallic glass after interrupted isothermal treatments at a temperature between glass transition and primary crystallization was studied by complementary techniques, namely atom probe tomography, transmission electron microscopy and X-ray diffractometry. During the early stages of crystallization of the glass, some nanometric (Fe,Nb)B crystals nucleate first by rejecting Y at their interface, which hinders their growth. The Fe62B14Y3 dendritic phase starts growing in a second step. Longer annealings lead to the development of chemical gradients in the amorphous matrix surrounding Fe62B14Y3 crystals, which favors the nucleation of additional nanometric (Fe,Nb)2.4B crystals.