It takes 12 hours for the body to use up its stores of ATP, locking the muscles. Highly doubtful that they were left out there that long but so much for Physiology and Legal Medicine.
Myosin levels may maxout in 12 hours, but rigor mortis starts to set in within 4
hours, in temperate climes
. High ambient temperatures can accelerate the
process. I'm not suggesting rigor caloris: I'm pointing out tropical conditions.
The encounter started at 6am, and the terrorists could have been killed very
early on, in an encounter that might have been brief. Securing the area for
postop assessment would have followed before the photo was taken, and
you can see the sun seems already very high, based on the shadows cast.