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.

At a news conference, Santa Fe County Sheriff Adan Mendoza said the initial examination by the medical examiner showed no sign of carbon monoxide, a colorless and odorless gas produced from kitchen appliances and other fuel-burning items. When it collects in poorly ventilated homes, it can be fatal.

Currently, authorities have not determined who died first, as autopsies are being conducted to establish the cause of their deaths.