The bodies of Gene Hackman, 95, and his 65-year-old wife Betsy were discovered inside their Santa Fe, N.M. home on Feb. 26.

It looked like both had been dead for days, authorities said.

Earlier this week, the Santa Fe County Sheriff’s Office confirmed there were “no significant findings” of gas leaks or carbon monoxide at their home.

So, what happened to the long-time couple remains unclear.

“Initially I was thinking carbon monoxide,” says James Gill, Chief Medical Examiner with the Connecticut Office of the Chief Medical Examiner, who is not investigating Hackman’s death in his professional capacity. “You’re thinking of something that all the people would be exposed to. But the fact that that’s negative then you start wondering about not only the cause of death, but the manner of death.”

Gill theorizes that from the initial circumstances it seems that Hackman may have collapsed. “He’s got a history of heart disease,” he says. “He’s got a pacemaker. So that would not be unusual. But the unusual part is that why then did she also collapse.”

“If she had found him alive, why didn’t she call 91 1? Which, you know, usually that’s a person’s first instinct,” adds Gill. “Maybe they had agreed that if he was found, he didn’t want to be resuscitated. She may have panicked and then either had a primary heart event herself, a cardiac event or maybe she got despondent, and she took pills.”

Gill says the medical examiner will also want to exclude that there were no injuries from a fall.

Toxicology and autopsy results, he says, should be able to exclude a drug-induced death or any injuries.

“I think the autopsy and the toxicology testing are really going to be what’s needed in order to make that final determination,” he says. “Otherwise, it’s kind of speculation at this point.”

Another possibility could be that Betsy died first and Gene “found her and was going out to get help or get his phone, and he then collapsed from the stress of that, too,” says Gill. “Just as likely. He’s older, he’s got known heart disease. But the autopsy would certainly show if she had heart disease or cancer or what have you.”

Authorities were called to the couple’s home after a pest control worker called a neighborhood security officer when he stopped by the couple’s home, and they didn’t answer. The neighborhood security officer called 911 when he saw the bodies through a window.

Hackman, who was dressed in gray sweatpants and a blue long-sleeved t-shirt, was found dead in the mudroom.  A pair of sunglasses and a walking cane were found nearby. An officer at the scene suspected that Hackman had “suddenly fallen,” according to a search warrant affidavit obtained by PEOPLE.

Betsy, 65, was found dead on the floor of the bathroom near a countertop. An officer noted a space heater near her head, suspecting that she may have knocked it over when she fell.

Also found on the counter were scattered pills.

The couple showed signs of decomposition and Betsy had bloating in her face and “mummification” in both hands and feet.

The couple’s dog was found dead about 10 to 15 feet from Betsy in a closet in the bathroom.