Spin injection from Co70Fe30 and Fe contacts into bulk GaAs(001) epilayers is studied experimentally. Using nonlocal measurements, the spin polarization of the differential conductance is determined as a function of the bias voltage applied across the injection interface. The spectra reveal an interface-related minority-spin peak at forward bias and a majority-spin peak at reverse bias and are very similar, but shifted in energy, for Co70Fe30 and for Fe contacts. An increase of the spin-injection efficiency and a shift of the spectrum correlate with the Ga-to-As ratio at the interface between CoFe and GaAs. © 2011 American Physical Society.