After he was reported missing, investigators "developed information" that led them to the property in Freeport, where Coulthart's body was recovered on June 25.
"This is not the news we hoped to deliver to our Niceville community," said Niceville Chief of Police Mark Hayse. "We stand with Jason's family and will continue working closely with our partners to see this case through."
Investigators believe Johnson was involved in Coulthart's death and have issued a warrant for her arrest.
She is believed to be "actively evading law enforcement," the sheriff's office said. Authorities have not disclosed her relationship with Coulthart.
Johnson could be using multiple aliases, including Harley, Jessica Elaine Bowman, Jessica Elaine Thrush, Jessica Dowdy, and others, authorities said.
Three other people have been arrested in connection with the killing. Michael Ray White, 74, and Kathleen Morris, 59, of Santa Rosa Beach, were arrested for accessory to a capital felony after the fact and obstruction. Investigators believe White knowingly helped Johnson evade law enforcement between June 25 and 30.
Morris was arrested after admitting she helped Johnson with transportation and supplies, as well as misleading law enforcement officials over Johnson being at her home, a homicide case. We will not allow anyone to get in the way of this investigation."
Authorities released surveillance video that captured Johnson inside a business, days after Coulthart's remains were discovered. She may have changed her appearance since, the sheriff's office said.