Gene Hackman and his wife, Betsy Arakawa, tested negative for carbon monoxide poisoning, the Santa Fe Sheriffs Department in New Mexico said Friday, one day after they were found dead with their dog.

Santa Fe County Sheriff Adan Mendoza told NBC's “Today” show that investigators were trying to sort out conflicting stories about the doors. He said several doors were unlocked and a rear door was open, which allowed two dogs that survived to go in and out. He also said he thinks the front door was closed but unlocked.

Gene Hackman and his wife Betsy Arakawa's death is just as much a surprise for their friends, including Barbara Lenihan, who spoke about it exclusively with PEOPLE.

See photos of the late Gene Hackman through the years in our tribute to the acclaimed actor's life and career, including his roles in films like 'Unforgiven,' 'The French Connection,' and 'The Royal Tenenbaums.

Gruff in demeanor yet much beloved, Gene Hackman was one of the finest actors of his generation. The 95-year-old actor was found dead in his New Mexico home with his wife, Betsy Arakawa, and their dog,