During our hike near Ajalpan, after the weather had cleared up a bit, we saw many large grey colored lizards running away from us. At that time I had no idea of what species I was looking at, but Tim Burkhardt identified the above lizard as Aspidoscelis sacki. The lizard was first in the genus Cnemidophorus but has been moved to the genus Aspidoscelis (Reeder et al. 2002). The common name for this species seems to be Sack's Giant Whiptail.
Near the end of our hike I got lucky and managed to make several photos of a very large specimen. Most likely moving very, very slow and hiding my eyes behind the Canon A640 point-and-shoot camera did the trick.
For another photo of this beautiful specimen, see: Sack's giant whiptail lizard (Aspidoscelis sacki)
This photo is part of the blog entry Police encounters, which has habitat photos and describes the environment.