This quote is from the AMA that Former Boulder Police Chief Mark Beckner conducted on Reddit a few years ago:
We know from the evidence she was hit in the head very hard with an unknown object, possibly a flashlight or similar type item. The blow knocked her into deep unconsciousness, which could have led someone to believe she was dead. The strangulation came 45 minutes to two hours after the head strike, based on the swelling on the brain. While the head wound would have eventually killed her, the strangulation actually did kill her. The rest of the scene we believe was staged, including the vaginal trauma, to make it look like a kidnapping/assault gone bad. I have avoided saying who I believe is responsible and let the facts speak for themselves. There are several viable theories.
This was part of the testimony that the Grand Jury heard about the head wound. I think it is most compelling when one considers that the coroner, Dr. John Meyer testified to the Grand Jury. Also testifying were Dr. Lucy Rorke, whom Dr. Meyer had asked to consult on the results of the autopsy specific to the head wound.
She was knocked deeply unconscious according to Dr. Rorke, and it is very possible that she was thought to be dead. Signs of life would have been difficult to detect. The brain is the "command center" of the body if you will. It is responsible for sending signals through the nervous system to the other organs to regulate things like breathing, heart rate, body temperature, digestion, etc. When someone is knocked unconscious, especially into a deep unconsciousness as was with JBR, organ function is affected. One of the things that happens is that blood pressure is diminished, and the first area to suffer from a loss of blood flow is the brain. Among the things noted by Dr. Rorke was that there was necrosis in the brain tissue. Necrosis is "the death of cells or tissues in the body that occurs when cells are deprived of oxygen, nutrients or blood flow, leading to their irreversible damage and eventual demise". In her deeply unconscious state, JBR's brain was being deprived of oxygen, nutrients and blood flow as a result of the lack of signals from the brain to the essential organs.
There is no indication in the autopsy report that Dr. Meyer cut away or shaved any hair from JBR's head to thoroughly examine the scalp. We do know from the lack of blood at the scene and on her head and in her hair that the skin of the scalp was not broken. But it is noted in the autopsy report that there was a scalp contusion. I would call that an injury.