Description: 

Overall: My first thought was that I was seeing an accipiter, probably a Cooper's Hawk as the bird was noticably smaller than the Turkey Vulture.  It appeared to be mostly unmarked from beneath except for easily discernable black primary tips.  The bird was very "broad-winged" and short-tailed suggesting a buteo rather than a Cooper's Hawk.

The bird was only seen from underneath.

Head: buteo shaped, broad and triangular, chin and throat  grayish-white
Wings: under-wing more or less uniformly grayish-white, primary tips black.
Breast: grayish-white
Belly and undertail coverts: grayish-white
Tail: grayish-white with faintly darker barring.

Given its apparent size relative to the nearby Turkey Vulture, most large buteos can be eliminated.  The bird we saw quite definitely did not have patagial marks.  Other details of the under-wing and tail also eliminate commonly occurring buteos, including the usually quite small looking  Red-shouldered Hawk.

Adult male Northern Harrier would also show unmarked underwings with black wing-tips, but the black is more extensive in Northern Harrier.  The tail has a much longer  look,  the wings look longer and narrower in proportion to the body and the banding in the tail, while quite pale on male harriers, would appear wider than what was seen on our bird.

Accipiters generally show a grayer under-body, more prominent tail bands and accipiter do not show black wing-tips.

Gray Hawk would be even less likely than Broad-wing, does not have dark, black wing-tips and should a more prominently banded tail.