Authorities in Washington state finally cracked a 51-year-old murder of a woman only to discover the suspect, a former sheriff's deputy, died three days before they were granted an arrest warrant.

The Spokane County Sheriff's Office revealed in a Facebook post this Monday that detectives were planning to arrest 85-year-old Duke Pierson in Covington County, Alabama in connection to the murder of Dorothy Fielding in 1967.

Fielding, who was 31 at the time, was reported missing on August 19, 1967. Her decomposed body was discovered eight months later in a shallow grave near the area of the 7-Mile Off Road Vehicle Park in Spokane.

"The case had remained unsolved since that time," the sheriff's office said.

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Two other deaths in the area around the same time caught the attention of police, who believed they might have been connected.

One was the case 47-year-old Ruby Lampson, who was reported missing on June 6, 1967. In 1971, Lampson’s "badly decomposed body" was discovered in a shallow grave near the 7-Mile ORV Park, not far from the location where Fielding’s remains were discovered, according to the sheriff's office.

The second was the reported suicide of 33-year-old Sandra Pierson, Duke Pierson’s wife at the time. Pierson's body was found in a vehicle in the family garage on Sept. 12, 1967, and there was a hose leading from the exhaust to a rear window. Tinfoil was found stuffed in the exhaust pipe of the vehicle with the garage door closed, but detectives discovered something strange.

"They noted the garage contained a lot of exhaust but found the engine of the vehicle was not running although the ignition was on and the gas tank was approximately 1/2 full," the sheriff's office said. "Detectives were unable to determine a reason why the engine was not running."

Sandra Pierson, who was 20-weeks pregnant at the time, died one month after Fielding's disappearance and 3-months after Lampson's disappearance.

"Due to Sandra’s small stature, several people including Sandra’s now adult children believe Sandra wasn’t physically capable of opening and closing the heavy, possibly malfunctioning, garage door on her own," police said. "Both children suspect Duke was involved in their mother’s death."

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After years of new no leads, officials reopened the case in April 2018 after the Major Crimes Unit received a call from someone who mentioned that Fielding was a member of the Falls View Tavern Bowling Team, which caught the attention of a Detective Kirk Keyser. Keyser later discovered that both Fielding and Lampson had gone to the tavern, and were friends.

When reviewing the Fielding investigation, he also found reports to indicate that she was having an affair with Duke Pierson before being reported missing, according to police.

"With these similarities, Detective Keyser began reviewing all three cold cases and worked to obtain additional information," police said. "This led to Friday’s arrest warrant for Pierson charging him with the murder of Fielding."

But that's when authorities discovered that Pierson died on Jan. 22, reportedly due to natural causes, according to the sheriff's department.

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Pierson had worked for the sheriff's office from 1959 before quitting in 1966. Former co-workers said he suddenly stopped showing up for work and his "his attitude and demeanor changed significantly."

"When coworkers contacted Duke to determine why he wasn’t coming to work, Duke threatened them, saying he would shoot/kill them, for no apparent reason," police said. "Duke was described as clearly being mentally unstable."

Keyser had gone to Pierson's home in Alabama in April 2018 and interviewed him, where he denied knowing Lampson or Fielding.

"Pierson admitted to dating thousands of women, and if he had dated Fielding, the relationship lasted no more than a week," police said.

The sheriff's office is asking anyone who may have any information about Duke or Sandra Pierson, Dorothy Fielding or Ruby Lampson to call Detective Keyser at 509-477-6611.