First, let's look at some of what we know:
The sheriff's office said Thursday afternoon that it has not determined a cause of death. Officials have said no signs of foul play are apparent, and an autopsy found no external trauma to either Hackman or Arakawa.
Toxicology and carbon monoxide tests were requested for both, and results are still pending, according to the sheriff's office.
Circumstances surrounding the death of the two deceased individuals to be suspicious enough in nature to require a thorough search and investigation,” according to the affidavit.
Deputies found Arakawa's body on the floor of a bathroom, according to the affidavit. A space heater was near her head, and an opened prescription bottle was on the counter with pills “scattered on the counter-top.” It was not immediately clear what kind of pills they were.
The deputies suspected "the heater could have fallen in the event the female abruptly fell to the ground."
They then found the dead dog, a German shepherd, about 10 to 15 feet from Arakawa in a closet in the bathroom.
In calling for a "thorough search and investigation," officials in the affidavit cited a number of oddities found at the residence: the unlocked front door, the space heater on the bathroom floor, the open pill bottle with scattered pills, the presence of two living dogs and a third that was dead, the fact that Hackman was in a separate room from Arakawa and no obvious signs of a gas leak.
First, there's no signs of external trauma, so that rules out quite a few possibilities.
Gene was 95, and according to google, only about 0.8% of the US population live that long (and they're mostly women) so he could've just expired naturally, or died because of the same thing that killed his wife, Since his wife was only 65 so her death must've been caused by 'something' since there's no mention of anything with regards to her health and at 85 she's too young to just die.
Based on the bathroom scene description, she might have been attempting to take a pill when she fell, and probably could not get up.
There are a number of things that could cause people to lose balance -- poisoning by CO or something else is definitely one possibility, though they could've also eaten or drunk something. In any case, two dead people and one dead dog in the same place is indicative that something unnatural may be behind the deaths.