The rate of advancement of rigor mortis is another method used to determine time of death. Rigor mortis is the stiffening of the muscles caused by chemical changes in the muscle tissue after death. The onset of rigor mortis normally begins within 2 to 4 hours after death and takes between 6 to 12 hours for the entire body to be affected. Normally after 24-36 hours after death, the affects of rigor mortis have dissipated.

According to the police report, JonBenet was last seen alive at approximately 10:00 p.m. on December 25, 1996. John Ramsey, in company with Fleet White, found JonBenet dead in the basement at approximately 1:05 p.m. on December 26, 1996. When police first sighted the body, they observed that the body was affected by advanced rigor mortis. Rigor mortis is known to spread through the lesser muscled parts of the body first and gradually spreads through the body affecting the larger body parts last. John Ramsey found JonBenet at 1:05 p.m. and her body was completely set with rigor mortis, which indicates that she had died between 10:00 p.m. on December 25 and 6:00 a.m. on December 26.

The police also reported a smell of decomposition on the body. Again, the rate of decomposition depends on room temperature and the body’s levels of bacteria and enzyme activity. Typically, for every ten-degree increase in room temperature, the rate of decomposition is doubled. For the odor of decomposition to have been detected by the police, JonBenet would have had to have died near the beginning of the estimated time frame. If that was the case, the perpetrator would have had ample time for the commission of the offense, write a ransom note and effect his or her escape."