Typical preceding heart rate changes, associated with hypovolemic shock, that are incompatable with life, could tell LE that the wearer was deceased at the time, and that the lack of recorded heart rate was not simply due to the FitBit being removed. Presumably this data, if available, would have been recovered from the FitBit server Cloud. There was no actual need for LE to physically possess the FitBit at the time the warrant was issued.
When you bleed out, as your overall blood volume and blood pressure falls, your heart rate will increase to compensate, until the circulating blood volume can't maintain oxygen perfusion of the tissues, even with compensating fluid compartment shifts. The heart rate will then "falter" as your BP crashes and you die, causing the heart rate to stop.
Also stab wounds can kill without a lot of external bleeding. A direct penetrating stab wound to the heart can cause a condition called "cardiac tamponade", with it's own characteristic pattern of tachycardia and sometimes dysrhythmias before death. In cardiac tamponade, the heart itself is pierced by the stab wound, and fills the non-elastic pericardial sac (that contains the heart) slowly compressing the heart and decreasing overall cardiac output, eventually causing death. How long to TOD after direct penetrating cardiac injury depends on the severity of the cardiac injury, and the penetrating object used, or if left in place. I have personally seen cardiac tamponade emergently addressed with a finger stuck directly into the heart, followed by a Foley urinary catheter which was then inflated, clamped shut, and pulled tight against the interior of the cardiac traumatic wound, with the catheter taped to the chest to hold tension and keep the hole successfully plugged until direct surgical intervention.
Please remember that "sharp force trauma" is a forensic mechanism of death, NOT a medical cause of death. "Cardiovascular collapse, secondary to an untreated, traumatic cardiac tamponade" IS a medical cause of death that would be suitable for a Death Certificate. The autopsy report findings released to the public are brief because of Iowa Law about public reporting in such matters, and because of medical and LE prudence. MT died as the result of injuries caused by sharp force trauma, per the public announcement by the State Medical Examiner's Office. This, again, is the mechanism that is felt to be the cause of death. 33 days of decomp in the elements can make it difficult to accurately and quickly determine the exact medical cause(s) of death.
Presumably a Homocide arrest warrant indicating an upper time range of 20:28 indicates that the swearing LEO had good reason to believe that the victim was dead by that time. It is a reasonable assumption that such a seemingly accurate time was not pulled out of thin air, but was determined from Cloud derived FitBit HR data. While NOT an EKG, the lowly FitBit can show diagnostic worthy rate and rhythm changes.
