December 11, 2017 07:40 PM

The killer moved in plain sight on the foggy morning of Sept. 6, 1982, in the tiny southeast Alaskan fishing village of Craig.

Hours after shooting his eight victims with what police believe was a .22-caliber pistol or rifle, he fired up the engines on the 58-foot fishing boat Investor, waving nonchalantly to a nearby skipper as he moved the vessel — with his victims’ bodies inside — to a secluded bay a mile outside of town.

He motored back to the weather-beaten docks in the Investor’s skiff, returning the next afternoon with a can of gasoline to set the craft ablaze before speeding back to town and then vanishing. Authorities would later describe the suspect as a white male in his early 20s with a pockmarked complexion.

“When I got there [to the boat], black smoke was coming out of the wheelhouse, but there was nobody on deck,” former Craig police chief Ray Shapley recalls in a story about the Investor killings in this week’s issue of PEOPLE. “It made the hair on the back of my neck stand up.”

Thirty-five years later, the slaying of skipper Mark Coulthurst and his pregnant wife, Irene, both 28, along with their children Kimberly, 5, and John, 4, and four deckhands — Chris Heyman, 18; and Jerome Keown, Dean Moon and Mike Stewart, all 19 — remains Alaska’s worst unsolved mass homicide.

The case will be featured on Monday night’s People Magazine Investigates on Investigation Discovery and the episode is exclusively previewed above.

• For more on this case, subscribe to PEOPLE or pick up this week’s issue, on newsstands now, and watch People Magazine Investigates: Murder at Sea on Monday, Dec. 11, at 10 p.m. ET on Investigation Discovery.

A painting of the doomed Investor
James Williamson
From left: Irene and Mark Coulthurst
Courtesy Laurie Hart
Chris Heyman

One of Coulthurst’s former deckhands, John Peel, was charged with the crimes in 1984. But after two headline-grabbing murder trials — reportedly the most expensive in state history — he was found not guilty of the homicides and Alaska police are no longer looking for the killer.

“The case is closed,” says Tim DeSpain, spokesman for the Alaska State Troopers.

“It was a pretty damn good investigation,” explains former Bellingham, Washington, police detective David McNeill, who helped Alaskan authorities investigate the case.

He continues, “Just because someone is acquitted doesn’t mean they’re innocent, just means there’s not enough evidence to show guilt beyond a reasonable doubt.”

Peel sued the state of Alaska for wrongful prosecution and received a $900,000 financial settlement. He calls the case “a tragedy that just got compounded and compounded.”

From left: Kimberly Coulthurst, John Coulthurst and Mike Stewart
Courtesy Laurie Hart
Jerome Keown (left) and Dean Moon
Dave Freeman

For those whose lives were upended by the killings, including Peel, the Investor slayings remain a painful, chilling cloud that refuses to lift.

“You never stop thinking about them,” says Dave Freeman, who grew up with deckhands Moon and Keown.

“The shock of losing everyone really tore up our town,” Freeman says.

Says Keown’s brother Brian: “I still get choked up.”

People Magazine Investigates: Murder at Sea airs Monday (10 p.m. ET) on Investigation Discovery.

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