  The VERITAS array of imaging atmospheric Cherenkov telescopes has collected
nearly 6000 hours of observations of active galactic nuclei (AGNs). It has
detected 39 very-high-energy (VHE, >100 GeV) AGNs at redshifts up to z = 0.9,
of which 24 are classified as high-frequency-peaked BL Lacertae objects (HBLs).
VERITAS has obtained an extensive dataset of HBL, including extreme HBL (xHBL),
observations, with lightcurves spanning up to 11 years, allowing the
characterization of their long-term and short-term variability. A study of
various xHBLs/HBLs in the VERITAS dataset is presented, and the correlation
with other energy bands tested. In particular, the short-term variability of
xHBLs as a function of energy within the VHE band is examined, exploring the
possibility that secondary gamma rays are produced in cosmic-ray interactions
with background photons.
