November 28, 2016 09:00 AM

More than 35 years after a teenage girl came home to find her family brutally slain in their resort cabin, police say the cold case is heating up. Subscribe now to PEOPLE, or pick up this week’s issue, on newsstands now, for more on this case.

The memory of the Keddie cabin murders that rocked his California community 35 years ago, when he was 16, has never left Plumas County Sheriff Greg Hagwood.

He personally knew the victims — all of whom were found bound, beaten or stabbed in April 1981. The summer before the slayings of Sue Sharp, 36; her son Johnny, 15; and Johnny’s 17-year-old friend Dana Wingate, Hagwood worked with the two boys on a painting crew.

Twelve-year-old Tina Sharp — who disappeared the night of the murders, and whose skeletal remains were found three years later — was a student of Hagwood’s mother, who was a teacher in his quiet hometown of Quincy, California, in the Sierra Nevada mountains.

The killings occurred seven miles down the road from Quincy, in the former resort setting of Keddie, where the Sharp family — single mom Sue, who had left a broken marriage, and her five kids — lived in cabin 28. A friend of Hagwood’s previously lived in that same cabin, and he tells PEOPLE he stayed there “probably a dozen times.”

Those factors played into Hagwood’s desire to reopen the cold case after he became Plumas sheriff in 2010. But he was moved even more by the desire to provide the surviving family with answers and to restore credibility to a department that had long drawn criticism — some alleging conspiracy — for its failure to solve the killings.

Keddie cabin 28 where the murders occurred in 1981.
Julian Dufort

“There were many, many years when little or nothing was done on [the Keddie] investigation,” Hagwood tells PEOPLE in this week’s issue, on newsstands now. “It’s obviously a worthwhile pursuit. There is not an expiration date on homicides, and to the extent that we have surviving siblings and family members, it is our fundamental obligation to understand who did this and why.”

Surrounded by designated national forest, Quincy and Keddie thrived when Plumas County benefited from bustling mining and timber industries — but declined as they did. The Keddie resort was on a downward slide when Sue Sharp moved there in 1980, joining an increasingly itinerant community.

The Sharp family’s status, and the decades that have passed since the murders, have forced Hagwood to justify his attention to the case in the face of those who ask, Why now?

“Especially in today’s environment, because somebody may be looked down upon as socially marginalized, economically marginalized …” he says, pausing. “There are people who lived in our community who rightfully deserved, and should expect, the same level of service and commitment.

“You cannot differentiate based on social status. If you do, you’ve forfeited the underlying principle of why we’re here.”

• Watch our new true crime show, People Magazine Investigates, which continues with an episode on the Keddie cabin murders on Monday at 10 p.m. ET on Investigation Discovery.

Courtesy Keddie28

Heating Up a Cold Case

Hagwood put the investigation’s review in motion in June 2013, when he brought back retired investigator Mike Gamberg to begin reorganizing the boxes of files and evidence that both say had been repeatedly shoved aside.

Gamberg, like Hagwood, has a personal tie to the case: As a martial arts instructor, he taught both Johnny Sharp and Dana Wingate, and Dana visited his home the day before he died.

“It struck this community harder than anything,” he remembers.

Although he worked for the sheriff at the time, Gamberg had not previously worked on the investigation. (Acknowledging his sometimes contentious personality, Gamberg volunteers that he was twice fired — and twice reinstated — under previous sheriffs.)

Starting over, Gamberg pored through testimony in the case about old theories, including rumored connections to drugs. “Sue Sharp was a quiet woman,” he says. “She was never that kind of individual. She was into her kids. She wasn’t a drinker or doper or anything like that; none of the things that people had theories about fit.”

RELATED VIDEO: 3 Things to Know About the Horrific Keddie Murders

“I think,” Gamberg says, “[Sue] was involved with one of the suspect’s wives, in that she was kind of counseling [the wife] about him and his abuse. I think this guy just went ballistic.”

That theory and suspect, Marty Smartt, again emerged front-and-center when Gamberg was alerted to a letter found in the case files, written by Marty to his then-wife, Marilyn, after the murders. “I’ve paid the price of your love & now that I’ve bought it with four peoples lives, you tell me we are through,” the letter says. “Great! What else do you want?”

(The former Marilyn Smartt tells PEOPLE she did not recall ever receiving the letter, which she says she learned about through investigators. But she did recognize Marty’s handwriting, she says.)

Gamberg also tracked down a therapist in Reno, Nevada, who says he heard a confession from Marty — and told Gamberg that he was surprised to learn investigators at the time, who the therapist says he alerted, didn’t use it to arrest him.

• Watch the full episode of the People Magazine Investigates After Show on the Keddie case, available Nov. 28 on People/Entertainment Weekly Network (PEN). Go to,  or download the app for Apple TV, Roku, Amazon Fire TV, Xumo, Chromecast, Xfinity, iOS and Android devices.

The scene of the Keddie cabin murders in April 1981.
Julian Dufort

‘Tangible Discoveries’

And then there is what Gamberg and Hagwood consider new evidence:

As news about the revived case circulated, Gamberg learned of a man with a metal detector who found a steel, blue-handled claw hammer in a pond near Keddie. That hammer, now recovered and being tested, exactly matched the description of one Marty said was missing when he was interviewed about the murders — which were carried out by a killer or killers using knives and hammers.

Further, Gamberg discovered in the case files a sealed envelope with the cassette of a 911 recording from an anonymous caller who claimed to know the remains found in the woods were Tina’s before that information was ever confirmed. The tape is now also being analyzed, to see if the voice matches recordings of any prior suspects or witnesses.

“Those are tangible discoveries,” Sheriff Hagwood tells PEOPLE. The digital voice comparisons represent “an opportunity that just hasn’t existed,” he says. “Why that that sat in a sealed evidence envelope, never opened, I don’t have the answer to that. But we have it now.”

Though Marty and an ex-con friend of his, “Bo” Boubede, who was living with him in Keddie at the time of the killings, were considered early suspects, Hagwood and Gamberg believe they’re close to identifying them as the killers. (Both men have since died.)

“It’s a theory that we are working, to the degree possible, to conclude or dismiss,” Hagwood says. “There’s a disproportionate amount of evidence and information that tends to pint in that direction.”

But they’re not stopping there.

Evidence from the Keddie cabin murders in April 1981.
Julian Dufort

New Persons of Interest?

“There are individuals … that they either participated after-the-fact or they have firsthand information,” Hagwood says.

“I believe we’ve identified some of them, and we know who they are, and we know where they are,” he says. The ongoing investigation “is going to lead us to them, and we’re coming.”

With at least six “persons of interest” identified, all of them still alive, he adds, “I believe they at minimum have firsthand information, and at a maximum participated in or assisted in destroying evidence and disposing of Tina’s remains.”

While that may not lead to charges, Gamberg says, it still offers resolution. “I really believe that it would give the family some kind of closure, if you would call it that. At least they will know, and maybe I will have confirmed their suspicions from the very beginning. That in itself is solving the case. They know that somebody never gave up on it.”

Sheila Sharp is indeed grateful. She was 14 when she discovered the bodies of her mother and brother in their cabin. Now 50, she tells PEOPLE, “In the last three years, he’s done more than the Plumas County Sheriff’s Office has done in the previous 32 years.”

From left: Plumas County sheriff's investigator Mike Gamberg and Sheila Sharp.
Julian Dufort

Hagwood says he’s not trying to validate the suggestions of anyone who alleges a possible cover-up by law enforcement

“That discussion will continue to be there,” he concedes. “We have to stay focused on what we’re doing now and how it will bring us closer to answering the question of who and why.”

But Hagwood acknowledges the work of online sleuths who have long attacked the perceived failings of prior investigations and, more recently, have provided paths for investigators to explore. “It has brought to light some amazing timelines, histories, and what some may call ‘coincidence’ and others may look at more accusingly,” he says.

As a result, “I don’t put anything outside the realm of possibility.”

Doug Thomas, the sheriff at the time the killings and who left for a new job with the state’s Department of Justice shortly afterward, acknowledges he’s a target of skeptics.

“There was no shortage of suspects,” Thomas tells PEOPLE. “But suddenly now everybody 35 years or so later have all figured out what happened, and that all of the investigating officers were corrupt. It’s laughable, is what it is.”

“Martin Smartt was not a friend of mine,” he says. “At one point he and his wife were having marital problems and they came to my office when I was sheriff and wanted me to counsel them. First of all, I had just gone through a divorce at that time. I told them, ‘Why would you want me to counsel you?’ “

But he did provide “one session” of advice, he says.

“That sounds strange, but anyhow,” Thomas says. He also dismisses the rumor that Marty had once stayed with him. “People’s imaginations go wild,” he says. “Other than that, there was no contact with them until the homicides.”

Marilyn says she does not recall any meeting between her, Marty and Thomas. Asked if Marty and Thomas were friends, she says, “Not to my knowledge.”

Placing blame matters less to Hagwood than finally reaching a conclusion.

“I knew these kids. I was the same age,” he says. “Now, 30-some years later I have the incredible good fortune to be the sheriff of Plumas County and revisit that which has a very interesting personal component.”

“Unfinished business,” he says, “is completely unacceptable.”

People Magazine Investigates‘ episode on the Keddie cabin murders airs Monday (10 p.m. ET) on Investigation Discovery.

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