Actor Gene Hackman, 95, and his wife, Betsy Arakawa, 65, both tested negative for carbon monoxide, authorities revealed Friday, amid an investigation after they were both mysteriously found dead alongside one of their dogs at their home in Santa Fe, New Mexico.

Santa Fe County Sheriff’s Office Adan Mendoza said he believes that carbon monoxide is ruled out as a possible cause of death.

Hackman is believed to have died on Feb. 17 — nine days before he and his wife were discovered dead — Mendoza also said Friday, noting that was the date of the last recorded “event” on his pacemaker. That is believed to have been Hackman’s “last day of life,” the sheriff said, noting that it is still unclear when Arakawa died.

A cause and manner of death remain pending, he said. Investigators are still awaiting full autopsy results and toxicology reports, he said.

The couple was found on Wednesday during a welfare check with no obvious signs of how they died, according to the Santa Fe County Sheriff’s Office.

However, their deaths were “suspicious enough in nature to require a thorough search and investigation” due to all of the “circumstances surrounding” the scene, according to the search warrant affidavit.

There was no external trauma to either of them, which led officials to conduct testing for carbon monoxide and toxicology, the sheriff’s office said.

Mendoza said the toxicology and autopsy results remain pending, and that it could be at least three months before they have the final autopsy findings.

He said the pathologist from the New Mexico Office of the Medical Investigator had expedited some tests and informed him earlier Friday that the couple had tested negative for carbon monoxide.

“She shared that information with me because she thought it was relevant to the case and important for the public to know,” Mendoza said.

In a Thursday search of the couple’s home, investigators recovered two cellphones, thyroid medication, blood pressure medication, Tylenol, a 2025 monthly planner and health records, court records show.

Hackman was discovered on the floor in the mud room, according to the search warrant. It appeared he fell suddenly, and he and his wife “showed obvious signs of death,” the document said.

Arakawa was found lying on her side on the floor in a bathroom, with a space heater near her body, according to the search warrant, and her body showed signs of decomposition due to some mummification to her hands and feet.

On the counter near Arakawa was an opened prescription bottle, with pills scattered, according to the search warrant.

A German shepherd was found dead about 10 to 15 feet from Arakawa, the document said. That dog was in a crate or a kennel, according to Mendoza. Two other dogs owned by the couple survived, though officials said both had access to a doggy door.

The Santa Fe City Fire Department found no signs of a possible carbon monoxide leak or poisoning, the document said. Tests found no carbon monoxide in the house, according to Fire Chief Brian Moya.

New Mexico Gas Company also responded, “As of now, there are no signs or evidence indicating there were any problems associated to the pipes in and around the residence,” the document said.

Two maintenance workers said they hadn’t heard from Hackman and Arakawa in about two weeks, the document said.

A maintenance worker who initially responded to the home found the front door open but there were no signs of forced entry or that anything had been stolen, the document said.

There was no indication of a crime and “there could be a multitude of reasons why the door was open,” the sheriff told reporters Thursday.

There was “no obvious sign or indication of foul play,” but authorities “haven’t ruled that out yet,” the sheriff said.

Investigators are “keeping everything on the table,” he added.

ABC News’ Kevin Shalvey, Erica Morris and Emily Shapiro contributed to this report.