Authorities investigating the deaths of Oscar-winner Gene Hackman and his wife said Friday they have found the two tested negative for carbon monoxide, but the cause of death is still unknown.

Santa Fe County Sheriff Aden Mendoza said he spoke with the pathologist from the New Mexico office of the medical investigator, who said that both Hackman and Arakawa tested negative for carbon monoxide.

The two apparently had been dead for days or even a couple of weeks when investigators found their bodies while searching the couple's Santa Fe home on Wednesday. Investigators are trying to figure out the last time anyone saw or spoke to them, Mendoza told NBC's “Today” show on Friday.

“That is a challenge because they were very private individuals,” the sheriff said, noting that the autopsy and toxicology testing results could take months.

Hackman, 95, was found Wednesday in an entryway of the home and Arakawa, 65, was found lying on her side in the bathroom. A dead German shepherd was found in a kennel near Arakawa, Mendoza said Thursday.

There was an open prescription bottle and pills on a countertop near Arakawa, according to the search warrant. According to court records filed Friday, authorities who searched the home retrieved medication that treats high blood pressure and chest pain, thyroid medication, Tylenol, two cellphones, a monthly planner and records from medical diagnostics testing.

There was no indication of foul play, according to the sheriff’s office. Detectives wrote in a search warrant affidavit that investigators thought the deaths were “suspicious enough in nature to require a thorough search and investigation.”

No gas leaks were discovered in and around the home, but a detective noted in the affidavit that people exposed to gas leaks or carbon monoxide might not show signs of poisoning.

A space heater was next to Arakawa and may have fallen when she abruptly fell to the floor, according to the affidavit. The sheriff’s office planned a Friday afternoon news conference to provide updates.

A maintenance worker who showed up to do routine work at the house discovered their bodies, investigators said. The worker said he was unable to get inside when a 911 operator asked whether the people in the house were breathing.

“I have no idea,” the subdivision’s caretaker said on the call. “I am not inside the house. It’s closed. It’s locked. I can’t go in. But I can see she’s laying down on the floor from the window.”

He and another worker later told authorities that they rarely saw the homeowners and that their last contact with them had been about two weeks ago.

Mendoza told “Today" there were several conflicting stories about which doors were locked at the house. Several were unlocked and a rear door was open, which allowed two dogs that survived to go in and out. He also said he thought the front door was closed but unlocked.

Hackman was among the most accomplished actors of his generation, appearing as villains, heroes and antiheroes in dozens of dramas, comedies and action films from the 1960s until his retirement in the early 2000s.

He was a five-time Oscar nominee who won best actor in a leading role for “The French Connection” in 1972 and best actor in a supporting role for “Unforgiven” two decades later. He also won praise for his role as a coach finding redemption in the sentimental favorite “Hoosiers.”

He met Arakawa, a classically trained pianist, at a California gym in the mid-1980s. They moved to Santa Fe by the end of the decade. Their Pueblo revival home, sits on a hill in a gated community with views of the foothills of the Rocky Mountains.

In his first couple of decades in New Mexico, Hackman was often seen around the state capital and served on the board of trustees for the Georgia O’Keeffe Museum from 1997-2004.

In recent years, he was far less visible. Aside from appearances at awards shows, he was rarely seen in the Hollywood social circuit and retired from acting about 20 years ago.

Hackman had three children from a previous marriage. He and Arakawa had no children but were known for having German shepherds.