Here are three pictures of a really huge, very creeply looking Robber Fly of the Mexican species.

I saw one of these in my yard last year and sent the pictures to www.whatsthatbug.com. Between them and www.bugguide.net they decided this is a:
"Mexican Robber Fly, Archilestris magnificus (Walker), a Mexican species which has been only a few times collected
in Arizon, but nowhere else in the US... It is not really a hanging thief, but it sure looks like
one."