  In this paper, we compile the very-high-energy and high-energy spectral
indices of 43 BL Lac objects from the literature. Based on a simple math model,
$\Delta\Gamma_{obs}=\alpha {\rm{z}}+\beta $, we present evidence for the origin
of an observed spectral break that is denoted by the difference between the
observed very-high-energy and high-energy spectral indices,
$\Delta\Gamma_{obs}$. We find by linear regression analysis that $\alpha\ne 0$
and $\beta\ne 0$. These results suggest that the extragalactic background light
attenuation and the intrinsic curvature dominate on the GeV-TeV $\gamma$-ray
energy spectral break of BL Lac objects. We argue that the extragalactic
background light attenuation is an exclusive explanation for the redshift
evolution of the observed spectral break.
