5 Things to Know about the Keddie Cabin Murders — and the New Hunt for the Killers

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 1981 Keddie cabin murders of three family members and a teenage friend shook the small Sierra Nevada mountain communities of Keddie and Quincy in northern California, beginning a mystery that lingers more than three decades later.

Who murdered the Sharp family?

Authorities once again are asking that question, reviving the investigation. Here are five things to know about their work.

1. The Scene Suggests More Than One Killer

On April 12, 1981, 14-year-old Sheila Sharp was returning from a sleepover when she discovered three dead bodies in her family’s cabin at the Keddie Resort: her 36-year-old single mother, Sue; her brother Johnny, 15; and Johnny’s friend Dana Wingate, 17. All had been bound and suffered extreme violence.

Sheila’s 12-year-old sister, Tina, was missing. The girl’s skull was found in the woods three years later, some 50 miles away in another county.

Adding to the mystery: Sheila’s younger brothers Greg and Rick, ages 5 and 10 at the time, were asleep and unharmed in an adjacent bedroom of the cabin, along with a friend, Justin Smartt.

According to the crime scene report, the boys’ bodies on the living room floor were bloodied around their heads and necks. Johnny was face-up, with hands and feet bound by electrical cord that also wrapped around Dana’s feet. Sue was covered by a yellow blanket; her hands and feet were bound by electrical wiring.

The three were killed using knives and a hammer. A bent steak knife lay on the floor, and a butcher knife and claw hammer, both also bloodied, were side-by-side on a small wooden table near the entry into the kitchen.

“Whoever did this — and there was more than one person — had to have blood all over them,” says Doug Thomas, now 76, who was the Plumas County sheriff at the time of the murders, leading the investigation, but who soon moved on to another job and has since retired.

• Watch our new true crime show, People Magazine Investigates, which continues with an episode on the Keddie cabin murders on Monday, Nov. 28, at 10 p.m. ET on Investigation Discovery.

2. The Primary Suspects Are Dead — But Authorities Are Still Pursuing Justice

Among those questioned in the killings was Sue’s neighbor Marty Smartt, who lived two cabins away with his wife (and Justin Smartt’s mother), Marilyn. It emerged that Marty was angry about Sue’s interference in his troubled marriage. Authorities also interviewed Marty’s ex-con friend “Bo” Boubede, who had moved in with Marty.

During the recently revived investigation, sheriff’s investigator Mike Gamberg was made aware of a letter written by Marty to Marilyn shortly after the crimes.

“I’ve paid the price of your love & now that I’ve bought it with four peoples lives, you tell me we are through,” Marty wrote. “Great! What else do you want?”

Marilyn, who later remarried, tells PEOPLE in this week’s issue, on newsstands now, that she doesn’t recall receiving the letter. She says she learned of it long afterward by investigators, but she recognized her ex-husband’s handwriting. Thomas, the retired sheriff, also says he heard about the letter only in recent months.

Gamberg further tracked down a therapist in Reno, Nevada, to whom Marty allegedly confessed after the killings — and who told Gamberg he was shocked to learn that investigators at the time didn’t use that confession to solve the crimes.

“There were some theories that Boubede was involved, and there was some theories that some other people in Keddie were involved,” Thomas says. Of Marty and Boubede, he says, “The only thing I knew is, they hung out together. We questioned them extensively.”

With nothing in those first weeks that tied them to the crime, Thomas says, neither were charged. Both left the area. Boubede died in 1988, and Marty Smartt died in 2006.

Yet Gamberg and current Sheriff Greg Hagwood believe the evidence leans toward those two as the culprits. “If a case needs to be pinned there,” Hagwood tells PEOPLE, “we’ll pin it.”

Thomas isn’t so sure. “It’s very easy to say somebody’s guilty when they’re dead,” he says. “They can’t defend themselves. I don’t think either one of them were involved.”

3. Investigators Have a Personal Stake in the Case

Keddie and Quincy are seven miles apart. In the larger community of Quincy, with a population of just under 2,000, Sheriff Hagwood was a schoolmate of victims Johnny and Dana, and the summer before the murders they’d all worked together as teenagers on a painting crew. Hagwood’s mother was a teacher who had Tina in class.

Hagwood also had a childhood friend who lived in Keddie’s Cabin 28 long before the Sharps moved in there. “I stayed in the cabin probably a dozen times,” he says.

Gamberg knew the boys as well. As a martial arts instructor at the time, he taught both Johnny and Dana, and says Dana was at his home the day before he died.

“It struck this community harder than anything,” he says of the killings.

• 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 PEOPLE.com/PEN, or download the app for Apple TV, Roku, Amazon Fire TV, Xumo, Chromecast, Xfinity, iOS and Android devices.

4. New Evidence Has Emerged

Hagwood became sheriff in 2010, and in June 2013 he brought Gamberg back from retirement to organize the boxes of Keddie reports and evidence with an eye toward reviving the case. Marty Smartt’s letter was an early re-discovery.

Just as important, Gamberg and Hagwood wonder if they’ve also recovered another of the murder weapons.

As reports of the renewed investigation went public, Gamberg learned of a man who, using a metal detector, had found a hammer in a pond near the Keddie property — and the steel, blue-handled claw hammer, now recovered, exactly matched the description of a hammer that Marty told investigators he’d lost. It is now being tested for trace DNA or blood residue.

Gamberg also found in the investigation’s files an apparently unopened envelope with a recording of a 911 call. When Tina’s remains were first found in 1984, on the third anniversary of the murders, her skull was unidentified. But that anonymous 911 caller said it was Tina’s before the medical examiner confirmed it through dental records.

“It’s my feeling that either he was told or he was involved in some way,” Gamberg says.

That tape, too, has been turned over for voice comparisons with audio of earlier suspects.

5. Investigators: People Can Confirm the Killers’ Identities

A vital thread, Gamberg says, is who knew about Tina’s remains placed in an isolated wooded area in neighboring Butte County — and who may have helped to put them there.

“There are people locally who know more than they’ve said,” Hagwood tells PEOPLE, “and I believe we’ve identified some of them, and we know who they are, and we know where they are. And I have every confidence that they either participated after the fact or they have first-hand information.”

“It’s obviously a worthwhile pursuit,” he says. “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 them to understand who did this and why.”

How close are they to solving the case?

“I don’t know,” he says. “I know we are closer than we’ve ever been.”

People Magazine Investigates‘ episode on the Keddie cabin murders airs Monday, Nov. 28 (10 p.m. ET), on Investigation Discovery.